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Pulsating low-mass white dwarf (WD) stars are WDs with stellar masses between 0.30 and 0.45 M⊙ that show photometric variability due to gravity-mode pulsations. Within this mass range, they can harbour both a helium core and hybrid core, depending if the progenitor experienced helium-core burning during the pre-WD evolution. SDSS J115219.99+024814.4 is an eclipsing binary system where both components are low-mass WDs, with stellar masses of 0.362 ± 0.014 M⊙ and 0.325 ± 0.013 M⊙. In particular, the less-massive component is a pulsating star, showing at least three pulsation periods of ∼1314, ∼1069, and ∼582.9 s. This opens the way to use asteroseismology as a tool to uncover its inner chemical structure, in combination with the information obtained using the light-curve modelling of the eclipses. To this end, using binary evolutionary models leading to helium- and hybrid-core WDs, we compute adiabatic pulsations for ℓ = 1 and ℓ = 2 gravity modes with Gyre. We found that the pulsating component of the SDSS J115219.99+024814.4 system must have a hydrogen envelope thinner than the value obtained from binary evolution computations, independently of the inner composition. Finally, from our asteroseismological study, we find a best-fitting model characterized by T |$_{\rm eff}=10\,917$| K, M = 0.338 M⊙, and M H = 10−6 M⊙ with the inner composition of a hybrid WD. [ABSTRACT FROM AUTHOR]

We have observed the eclipsing, post-common envelope white dwarf–brown dwarf binary, SDSS141126.20+200911.1, in the near-IR with the HAWK-I imager, and present here the first direct detection of the dark side of an irradiated brown dwarf in the H band, and a tentative detection in the K sband. Our analysis of the light curves indicates that the brown dwarf is likely to have an effective temperature of 1300 K, which is not consistent with the effective temperature of 800 K suggested by its mass and radius. As the brown dwarf is already absorbing almost all the white dwarf emission in the Ks band, we suggest that this inconsistency may be due to the UV-irradiation from the white dwarf inducing an artificial brightening in the K sband, similar to that seen for the similar system WD0137-349B, suggesting this brightening may be characteristic of these UV-irradiated binaries. [ABSTRACT FROM AUTHOR]

We analysed recent K2 data of the short-period eclipsing binary system HW Vir, which consists of a hot subdwarf-B type primary with an M-dwarf companion. We determined the mid-times of eclipses, calculated O–C diagrams, and an average shift of the secondary minimum. Our results show that the orbital period is stable within the errors over the course of the 70 days of observations. Interestingly, the offset from mid-orbital phase between the primary and the secondary eclipses is found to be 1.62 s. If the shift is explained solely by light-travel time, the mass of the sdB primary must be 0.26 M⊙, which is too low for the star to be core-helium burning. However, we argue that this result is unlikely to be correct and that a number of effects caused by the relative sizes of the stars conspire to reduce the effective light-travel time measurement. After removing the flux variation caused by the orbit, we calculated the amplitude spectrum to search for pulsations. The spectrum clearly shows periodic signal from close to the orbital frequency up to 4600 µHz, with the majority of peaks found below 2600 µHz. The amplitudes are below 0.1 part-per-thousand, too low to be detected with ground-based photometry. Thus, the high-precision data from the Kepler spacecraft has revealed that the primary of the HW Vir system is a pulsating sdBV star. We argue that the pulsation spectrum of the primary in HW Vir differs from that in other sdB stars due to its relatively fast rotation that is (nearly) phase-locked with the orbit. [ABSTRACT FROM AUTHOR]

M dwarfs are prime targets in the hunt for habitable worlds around other stars. This is due to their abundance as well as their small radii and low masses and temperatures, which facilitate the detection of temperate, rocky planets in orbit around them. However, the fundamental properties of M dwarfs are difficult to constrain, often limiting our ability to characterize the planets they host. Here we test several theoretical relationships for M dwarfs by measuring 23 high-precision, model-independent masses and radii for M dwarfs in binaries with white dwarfs. We find a large scatter in the radii of these low-mass stars, with 25 per cent having radii consistent with theoretical models while the rest are up to 12 per cent overinflated. This scatter is seen in both partially and fully convective M dwarfs. No clear trend is seen between the overinflation and age or metallicity, but there are indications that the radii of slowly rotating M dwarfs are more consistent with predictions, albeit with a similar amount of scatter in the measurements compared to more rapidly rotating M dwarfs. The sample of M dwarfs in close binaries with white dwarfs appears indistinguishable from other M dwarf samples, implying that common envelope evolution has a negligible impact on their structure. We conclude that theoretical and empirical mass–radius relationships lack the precision and accuracy required to measure the fundamental parameters of M dwarfs well enough to determine the internal structure and bulk composition of the planets they host. [ABSTRACT FROM AUTHOR]

AM CVn-type systems are ultracompact, hydrogen-deficient accreting binaries with degenerate or semidegenerate donors. The evolutionary history of these systems can be explored by constraining the properties of their donor stars. We present high-speed photometry of Gaia14aae, an AM CVn with a binary period of 49. 7 min and the first AM CVn in which the central white dwarf is fully eclipsed by the donor star. Modelling of the light curves of this system allows for the most precise measurement to date of the donor mass of an AM CVn, and relies only on geometric and well-tested physical assumptions. We find a mass ratio q = M2/M1 = 0.0287 ± 0.0020 and masses M1 = 0.87 ± 0.02 M⊙ and M2 = 0.0250 ± 0.0013 M⊙. We compare these properties to the three proposed channels for AM CVn formation. Our measured donor mass and radius do not fit with the contraction that is predicted for AM CVn donors descended from white dwarfs or helium stars at long orbital periods. The donor properties we measure fall in a region of parameter space in which systems evolved from hydrogen-dominated cataclysmic variables are expected, but such systems should show spectroscopic hydrogen, which is not seen in Gaia14aae. The evolutionary history of this system is therefore not clear. We consider a helium-burning star or an evolved cataclysmic variable to be the most likely progenitors, but both models require additional processes and/or fine-tuning to fit the data. Additionally, we calculate an updated ephemeris which corrects for an anomalous time measurement in the previously published ephemeris. [ABSTRACT FROM AUTHOR]

Using data from the extended Kepler mission in K2 Campaign 10, we identify two eclipsing binaries containing white dwarfs with cool companions that have extremely short orbital periods of only 71.2 min (SDSS J1205−0242, a.k.a. EPIC 201283111) and 72.5 min (SDSS J1231+0041, a.k.a. EPIC 248368963). Despite their short periods, both systems are detached with small, low-mass companions, in one case a brown dwarf and in the other case either a brown dwarf or a low-mass star. We present follow-up photometry and spectroscopy of both binaries, as well as phase-resolved spectroscopy of the brighter system, and use these data to place preliminary estimates on the physical and binary parameters. SDSS J1205−0242 is composed of a 0.39 ± 0.02Mʘ helium-core white dwarf that is totally eclipsed by a 0.049 ± 0.006Mʘ (51 ± 6MJ) brown-dwarf companion, while SDSS J1231+0041 is composed of a 0.56 ± 0.07Mʘ white dwarf that is partially eclipsed by a companion of mass ≤0.095 Mʘ. In the case of SDSS J1205−0242, we look at the combined constraints from common-envelope evolution and brown-dwarf models; the system is compatible with similar constraints from other post-common-envelope binaries, given the current parameter uncertainties, but has potential for future refinement. [ABSTRACT FROM AUTHOR]

We present high-precision, model-independent, mass and radius measurements for 16 white dwarfs in detached eclipsing binaries and combine these with previously published data to test the theoretical white dwarf mass-radius relationship.We reach a mean precision of 2.4 per cent in mass and 2.7 per cent in radius, with our best measurements reaching a precision of 0.3 per cent in mass and 0.5 per cent in radius.We find excellent agreement between the measured and predicted radii across a wide range of masses and temperatures. We also find the radii of all white dwarfs with masses less than 0.48 M☉ to be fully consistent with helium core models, but they are on average 9 per cent larger than those of carbon-oxygen core models. In contrast, white dwarfs with masses larger than 0.52 M☉ all have radii consistent with carbon-oxygen core models. Moreover, we find that all but one of the white dwarfs in our sample have radii consistent with possessing thick surface hydrogen envelopes (10-5 ≥ MH/MWD ≥ 10-4), implying that the surface hydrogen layers of these white dwarfs are not obviously affected by common envelope evolution. [ABSTRACT FROM AUTHOR]

We present high-speed, multicolour photometry of the faint, eclipsing cataclysmic variable (CV) SDSS J105754.25+275947.5. The light from this system is dominated by the white dwarf. Nonetheless, averaging many eclipses reveals additional features from the eclipse of the bright spot. This enables the fitting of a parametrized eclipse model to these average light curves, allowing the precise measurement of system parameters. We find a mass ratio of q = 0.0546 ± 0.0020 and inclination i = 85.74 ± 0.21°. The white dwarf and donor masses were found to be Mw = 0.800 ±0.015Mʘ and Md = 0.0436 ±0.0020Mʘ, respectively. A temperature Tw = 13300 ± 1100 K and distance d = 367 ± 26 pc of the white dwarf were estimated through fitting model atmosphere predictions to multicolour fluxes. The mass of the white dwarf in SDSS 105754.25+275947.5 is close to the average for CV white dwarfs, while the donor has the lowest mass yet measured in an eclipsing CV. A low-mass donor and an orbital period (90.44 min) significantly longer than the period minimum strongly suggest that this is a bona fide period-bounce system, although formation from a white dwarf/brown dwarf binary cannot be ruled out. Very few period-minimum/period-bounce systems with precise system parameters are currently known, and as a consequence the evolution of CVs in this regime is not yet fully understood. [ABSTRACT FROM AUTHOR]

We present a long-term programme for timing the eclipses of white dwarfs in close binaries to measure apparent and/or real variations in their orbital periods. Our programme includes 67 close binaries, both detached and semi-detached and with M-dwarfs, K-dwarfs, brown dwarfs or white dwarfs secondaries. In total, we have observed more than 650 white dwarf eclipses. We use this sample to search for orbital period variations and aim to identify the underlying cause of these variations. We find that the probability of observing orbital period variations increases significantly with the observational baseline. In particular, all binaries with baselines exceeding 10 yr, with secondaries of spectral type K2 - M5.5, show variations in the eclipse arrival times that in most cases amount to several minutes. In addition, among those with baselines shorter than 10 yr, binaries with late spectral type (>M6), brown dwarf or white dwarf secondaries appear to show no orbital period variations. This is in agreement with the so-called Applegate mechanism, which proposes that magnetic cycles in the secondary stars can drive variability in the binary orbits. We also present new eclipse times of NN Ser, which are still compatible with the previously published circumbinary planetary system model, although only with the addition of a quadratic term to the ephemeris. Finally, we conclude that we are limited by the relatively short observational baseline for many of the binaries in the eclipse timing programme, and therefore cannot yet draw robust conclusions about the cause of orbital period variations in evolved, white dwarf binaries. [ABSTRACT FROM AUTHOR]

We present high-speed photometry and high-resolution spectroscopy of the eclipsing post-common-envelope binary QS Virginis (QS Vir). Our Ultraviolet and Visual Echelle Spectrograph (UVES) spectra span multiple orbits over more than a year and reveal the presence of several large prominences passing in front of both the M star and its white dwarf companion, allowing us to triangulate their positions. Despite showing small variations on a time-scale of days, they persist for more than a year and may last decades. One large prominence extends almost three stellar radii from the M star. Roche tomography reveals that the M star is heavily spotted and that these spots are long-lived and in relatively fixed locations, preferentially found on the hemisphere facing the white dwarf. We also determine precise binary and physical parameters for the system. We find that the 14220 ± 350 K white dwarf is relatively massive, 0.782 ± 0.013M⊙, and has a radius of 0.01068 ± 0.00007R⊙, consistent with evolutionary models. The tidally distorted M star has a mass of 0.382 ± 0.006M⊙ and a radius of 0.381 ± 0.003R⊙, also consistent with evolutionary models. We find that the magnesium absorption line from the white dwarf is broader than expected. This could be due to rotation (implying a spin period of only ~700 s), or due to a weak (~100 kG) magnetic field, we favour the latter interpretation. Since the M star's radius is still within its Roche lobe and there is no evidence that it is overinflated, we conclude that QS Vir is most likely a pre-cataclysmic binary just about to become semidetached. [ABSTRACT FROM AUTHOR]

SDSS J1021+1744 is a detached, eclipsing white dwarf/M dwarf binary discovered in the Sloan Digital Sky Survey (SDSS). Outside the primary eclipse, the light curves of such systems are usually smooth and characterized by low-level variations caused by tidal distortion and heating of theMstar component. Early data on SDSS J1021+1744 obtained in 2012 June were unusual in showing a dip in flux of uncertain origin shortly after the white dwarf's eclipse. Here we present high-time resolution, multiwavelength observations of 35 more eclipses over 1.3 yr, showing that the dip has a lifetime extending over many orbits. Moreover the 'dip' is in fact a series of dips that vary in depth, number and position, although they are always placed in the phase interval 1.06-1.26 after the white dwarf's eclipse, near the L5 point in this system. Since SDSS J1021+1744 is a detached binary, it follows that the dips are caused by the transit of the white dwarf by material around the Lagrangian L5 point. A possible interpretation is that they are the signatures of prominences, a phenomenon already known from H a observations of rapidly rotating single stars as well as binaries. What makes SDSS J1021+1744 peculiar is that the material is dense enough to block continuum light. The dips appear to have finally faded out around 2015 May after the first detection by Parsons et al. in 2012, suggesting a lifetime of years. [ABSTRACT FROM AUTHOR]

We present high-speed, three-colour photometry of the eclipsing dwarf nova PHL 1445, which, with an orbital period of 76.3 min, lies just below the period minimum of ~82 min for cataclysmic variable stars (CVs). Averaging four eclipses reveals resolved eclipses of the white dwarf and bright spot. We determined the system parameters by fitting a parametrized eclipse model to the averaged light curve. We obtain a mass ratio of q = 0.087 ± 0.006 and inclination i = 85°.2 ± 0. ? 9. The primary and donor masses were found to be Mw = 0.73 ± 0.03M⊙ and Md = 0.064 ± 0.005M⊙, respectively. Through multicolour photometry a temperature of the white dwarf of Tw = 13 200 ± 700 K and a distance of 220 ± 50 pc were determined. The evolutionary state of PHL 1445 is uncertain. We are able to rule out a significantly evolved donor, but not one that is slightly evolved. Formation with a brown dwarf donor is plausible, though the brown dwarf would need to be no older than 600 Myr at the start of mass transfer, requiring an extremely low mass ratio (q = 0.025) progenitor system. PHL 1445 joins SDSS 1433 as a sub-period minimum CV with a substellar donor. The existence of two such systems raises an alternative possibility that current estimates for the intrinsic scatter and/or position of the period minimum may be in error. [ABSTRACT FROM AUTHOR]

We report on the search for new eclipsing white dwarf plus main-sequence (WDMS) binaries in the light curves of the Catalina surveys. We use a colour-selected list of almost 2000 candidate WDMS systems from the Sloan Digital Sky Survey, specifically designed to identify WDMS systems with cool white dwarfs and/or early M-type main-sequence stars. We identify a total of 17 eclipsing systems, 14 of which are new discoveries. We also find three candidate eclipsing systems, two main-sequence eclipsing binaries and 22 non-eclipsing close binaries. Our newly discovered systems generally have optical fluxes dominated by the main-sequence components, which have earlier spectral types than the majority of previously discovered eclipsing systems. We find a large number of ellipsoidally variable binaries with similar periods, near 4 h, and spectral types M2-3, which are very close to Roche lobe filling. We also find that the fraction of eclipsing systems is lower than found in previous studies and likely reflects a lower close binary fraction among WDMS binaries with early M-type main-sequence stars due to their enhanced angular momentum loss compared to fully convective late M-type stars, hence causing them to become cataclysmic variables quicker and disappear from the WDMS sample. Our systems bring the total number of known detached, eclipsing WDMS binaries to 71. [ABSTRACT FROM AUTHOR]

We present Hubble Space Telescope + Cosmic Origins Spectrograph (HST+COS) data of the eclipsing double white dwarf binary CSS 41177. Due to the temperature difference between the two white dwarfs, the HST+COS far-ultraviolet data are dominated by the hot, primary white dwarf and allow us to precisely measure its temperature (T1). Using eclipse observations, we also tightly constrain the temperature of the cooler secondary white dwarf (T2). Our results, where T1 = 22 439 ± 59 K and T2 = 10 876 ± 32 K, with the uncertainties being purely statistical, place the secondary inside and close to the blue edge of the empirical instability strip for low-temperature hydrogen-atmosphere white dwarfs. Dedicated high-speed photometry is encouraged to probe for the presence of pulsations, which will constrain the border of the instability strip as well as probe a new region of low gravity within the strip. [ABSTRACT FROM PUBLISHER]

We present the first results of a dedicated search for pulsating white dwarfs (WDs) in detached WD plus main-sequence (MS) binaries. Candidate systems were selected from a catalogue of WD+MS binaries, based on the surface gravities and effective temperatures of the WDs. We observed a total of 26 systems using ULTRACAM mounted on ESO's 3.5 m New Technology Telescope at La Silla. Our photometric observations reveal pulsations in seven WDs of our sample, including the first pulsating WD with an MS companion in a post-common envelope (CE) binary, SDSS J1136+0409. Asteroseismology of these new pulsating systems will provide crucial insight into how binary interactions, particularly the CE phase, affect the internal structure and evolution of WDs. In addition, our observations have revealed the partially eclipsing nature of one of our targets, SDSS J1223-0056. [ABSTRACT FROM AUTHOR]

We present high time resolution SDSS-g' and SDSS-z' light curves of the primary eclipse in SDSS J141126.20+200911.1, together with time-resolved X-Shooter spectroscopy and near-infrared (NIR) JHKs photometry. Our observations confirm the substellar nature of the companion, making SDSS J141126.20+200911.1 the first eclipsing white dwarf/brown dwarf binary known. We measure a (white dwarf model dependent) mass and radius for the brown dwarf companion of M2 = 0.050 ± 0.002 M⊙ and R2 = 0.072 ± 0.004 M⊙, respectively. The lack of a robust detection of the companion light in the z'-band eclipse constrains the spectral type of the companion to be later than L5. Comparing the NIR photometry to the expected white dwarf flux reveals a clear Ks-band excess, suggesting a spectral type in the range L7–T1. The radius measurement is consistent with the predictions of evolutionary models, and suggests a system age in excess of 3 Gyr. The low companion mass is inconsistent with the inferred spectral type of L7–T1, instead predicting a spectral type nearer T5. This indicates that irradiation of the companion in SDSS J141126.20+200911.1 could be causing a significant temperature increase, at least on one hemisphere. [ABSTRACT FROM AUTHOR]

We present new eclipse observations of the polar (i.e. semidetached magnetic white dwarf + M-dwarf binary) HU Aqr, and mid-egress times for each eclipse, which continue to be observed increasingly early. Recent eclipses occurred more than 70 s earlier than the prediction from the latest model that invoked a single circumbinary planet to explain the observed orbital period variations, thereby conclusively proving this model to be incorrect. Using ULTRACAM data, we show that mid-egress times determined for simultaneous data taken at different wavelengths agree with each other. The large variations in the observed eclipse times cannot be explained by planetary models containing up to three planets, because of poor fits to the data as well as orbital instability on short time-scales. The peak-to-peak amplitude of the O-C diagram of almost 140 s is also too great to be caused by Applegate's mechanism, movement of the accretion spot on the surface of the white dwarf, or by asynchronous rotation of the white dwarf. What does cause the observed eclipse time variations remains a mystery. [ABSTRACT FROM AUTHOR]

We analyse the Catalina Real-time Transient Survey light curves of 835 spectroscopically confirmed white dwarf plus main-sequence binaries from the Sloan Digital Sky Survey (SDSS) with g < 19, in search of new eclipsing systems. We identify 29 eclipsing systems, 12 of which were previously unknown. This brings the total number of eclipsing white dwarf plus main-sequence binaries to 49. Our set of new eclipsing systems contains two with periods of 1.9 and 2.3 d, making them the longest period eclipsing white dwarf binaries known. We also identify one system which shows very large ellipsoidal modulation (almost 0.3 mag), implying that the system is both very close to Roche lobe overflow and at high inclination. However, our follow-up photometry failed to firmly detect an eclipse, meaning that either this system contains a cool white dwarf and hence the eclipse is very shallow and undetectable in our red-sensitive photometry or that it is non-eclipsing. Radial velocity measurements for the main-sequence stars in three of our newly identified eclipsing systems imply that their white dwarf masses are lower than those inferred from modelling their SDSS spectra. 13 non-eclipsing post-common envelope binaries were also identified, from either reflection or ellipsoidal modulation effects. The white dwarfs in our newly discovered eclipsing systems span a wide range of parameters, including low-mass (∼0.3 M⊙), very hot (80 000 K) and a DC white dwarf. The spectral types of the main-sequence stars range from M2 to M6. This makes our sample ideal for testing white dwarf and low-mass star mass–radius relationships as well as close binary evolution. [ABSTRACT FROM AUTHOR]

ABSTRACT Studies of cool white dwarfs in the solar neighbourhood have placed a limit on the age of the Galactic disc of 8-9 billion years. However, determining their cooling ages requires the knowledge of their effective temperatures, masses, radii and atmospheric composition. So far, these parameters could only be inferred for a small number of ultracool white dwarfs for which an accurate distance is known, by fitting their spectral energy distributions in conjunction with a theoretical mass-radius relation. However, the mass-radius relation remains largely untested, and the derived cooling ages are hence model dependent. Here we report direct measurements of the mass and radius of an ultracool white dwarf in the double-lined eclipsing binary SDSS J013851.54−001621.6. We find MWD = 0.529 ± 0.010 M⊙ and RWD = 0.0131 ± 0.0003 R⊙. Our measurements are consistent with the mass-radius relation and we determine a robust cooling age of 9.5 billion years for the 3570 K white dwarf. We find that the mass and radius of the low-mass companion star, Msec = 0.132 ± 0.003 M⊙ and Rsec = 0.165 ± 0.001 R⊙, are in agreement with evolutionary models. We also find evidence that this >9.5 Gyr old M5 star is still active, far beyond the activity lifetime for a star of its spectral type. This is likely caused by the high tidally enforced rotation rate of the star. The companion star is close to filling its Roche lobe and the system will evolve into a cataclysmic variable in only 70 Myr. Our direct measurements demonstrate that this system can be used to calibrate ultracool white dwarf atmospheric models. [ABSTRACT FROM AUTHOR]

ABSTRACT We present time-resolved spectroscopy of the eclipsing, short-period cataclysmic variable CTCV J1300−3052. Using absorption features from the secondary star, we determine the radial velocity semi-amplitude of the secondary star to be K2= 378 ± 6 km s−1, and its projected rotational velocity to be v sin i= 125 ± 7 km s−1. Using these parameters and Monte Carlo techniques, we obtain masses of M1= 0.79 ± 0.05 M⊙ for the white dwarf primary and M2= 0.198 ± 0.029 M⊙ for the M-type secondary star. These parameters are found to be in good agreement with previous mass determinations found via photometric fitting techniques, supporting the accuracy and validity of photometric mass determinations in short-period CVs. [ABSTRACT FROM AUTHOR]

We present high-speed ULTRACAM photometry of the eclipsing post-common-envelope binaries DE CVn, GK Vir, NN Ser, QS Vir, RR Cae, RX J , SDSS and SDSS and use these data to measure precise mid-eclipse times in order to detect any period variations. We detect a large (∼250 s) departure from linearity in the eclipse times of QS Vir which Applegate's mechanism fails to reproduce by an order of magnitude. The only mechanism able to drive this period change is a third body in a highly elliptical orbit. However, the planetary/sub-stellar companion previously suggested to exist in this system is ruled out by our data. Our eclipse times show that the period decrease detected in NN Ser is continuing, with magnetic braking or a third body the only mechanisms able to explain this change. The planetary/sub-stellar companion previously suggested to exist in NN Ser is also ruled out by our data. Our precise eclipse times also lead to improved ephemerides for DE CVn and GK Vir. The width of a primary eclipse is directly related to the size of the secondary star and variations in the size of this star could be an indication of Applegate's mechanism or Wilson (starspot) depressions which can cause jitter in the O C curves. We measure the width of primary eclipses for the systems NN Ser and GK Vir over several years but find no definitive variations in the radii of the secondary stars. However, our data are precise enough to show the effects of Applegate's mechanism in the future. We find no evidence of Wilson depressions in either system. We also find tentative indications that flaring rates of the secondary stars depend on their mass rather than rotation rates. [ABSTRACT FROM AUTHOR]

Using the high resolution Ultraviolet and Visual Echelle Spectrograph (UVES) mounted on the Very Large Telescope in combination with photometry from the high-speed CCD camera ULTRACAM, we derive precise system parameters for the pre-cataclysmic binary NN Ser. A model fit to the ULTRACAM light curves gives the orbital inclination as and the scaled radii, and . Analysis of the He ii 4686 Å absorption line gives a radial velocity amplitude for the white dwarf of . We find that the irradiation-induced emission lines from the surface of the secondary star give a range of observed radial velocity amplitudes due to differences in optical depths in the lines. We correct these values to the centre of mass of the secondary star by computing line profiles from the irradiated face of the secondary star. We determine a radial velocity of , with an error dominated by the systematic effects of the model. This leads to a binary separation of a , radii of and and masses of and . The masses and radii of both components of NN Ser were measured independently of any mass–radius relation. For the white dwarf, the measured mass, radius and temperature show excellent agreement with a ‘thick’ hydrogen layer of fractional mass . The measured radius of the secondary star is 10 per cent larger than predicted by models, however, correcting for irradiation accounts for most of this inconsistency, hence the secondary star in NN Ser is one of the first precisely measured very low mass objects to show good agreement with models. ULTRACAM and z′ photometry taken during the primary eclipse determines the colours of the secondary star as and which corresponds to a spectral type of . This is consistent with the derived mass, demonstrating that there is no detectable heating of the unirradiated face, despite intercepting radiative energy from the white dwarf which exceeds its own luminosity by over a factor of 20. [ABSTRACT FROM AUTHOR]