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The Crab pulsar emits across a large part of the electromagnetic spectrum. Determining the time delay between the emission at different wavelengths will allow to better constrain the site and mechanism of the emission. We have simultaneously observed the Crab Pulsar in the optical with S-Cam, an instrument based on Superconducting Tunneling Junctions (STJs) with $\mu$s time resolution and at 2 GHz using the Nan\c{c}ay radio telescope with an instrument doing coherent dedispersion and able to record giant pulses data. We have studied the delay between the radio and optical pulse using simultaneously obtained data therefore reducing possible uncertainties present in previous observations. We determined the arrival times of the (mean) optical and radio pulse and compared them using the tempo2 software package. We present the most accurate value for the optical-radio lag of 255 $\pm$ 21 $\mu$s and suggest the likelihood of a spectral dependence to the excess optical emission asociated with giant radio pulses., Comment: 8 pages; accepted for publication in Astronomy and Astrophysics

We have observed the Crab Pulsar in the optical with S-Cam, an instrument based on Superconducting Tunneling Junctions (STJs) with $\mu$s time resolution. Our aim was to study the delay between the radio and optical pulse. The Crab Pulsar was observed three times over a time span of almost 7 years, on two different locations, using three different versions of the instrument, and using two different GPS units. We consistently find that the optical peak leads the radio peak by 49$\pm$90, 254$\pm$170, and 291$\pm$100 $\mu$s. On assumption of a constant optical lead, the weighted-average value is $\sim$170 $\mu$s, or when rejecting (based on a perhaps questionable radio ephemeris) the first measurement, 273$\pm$100 $\mu$s., Comment: 5 pages, 2 figures, accepted for publication in Astronomy & Astrophysics

We report the discovery of narrow X-ray absorption lines from the low-mass X-ray binary MXB1659-298 during an XMM-Newton observation in 2001 February. The 7.1 hr orbital cycle is clearly evident with narrow X-ray eclipses preceded by intense dipping activity. A sinusoid-like OM $B$-band modulation with a peak-to-peak modulation of 0.5 magnitude and a minimum coincident with the X-ray eclipse is visible. EPIC and RGS spectra reveal the presence of narrow resonant absorption features identified with OVIII 1s-2p, 1s-3p and 1s-4p, NeX 1s-2p, FeXXV 1s-2p, and FeXXVI 1s-2p transitions, together with a broad Fe emission feature at ~6.5 keV. The EWs of the Fe absorption features show no obvious dependence on orbital phase, even during dipping intervals. Previously, the only X-ray binaries known to exhibit narrow X-ray absorption lines were two superluminal jet sources and it had been suggested that these features are related to the jet formation mechanism. This now appears unlikely, and instead their presence may be related to the viewing angle of the system. The MXB1659-298 0.6--12 keV continuum is modeled using absorbed cutoff power-law and blackbody components. During dips the blackbody is more strongly absorbed than the power-law. The spectral shape of the 3.6% of 0.5--10 keV emission that remains during eclipses is consistent with that during non-dipping intervals., Comment: 11 pages, 10 figures, Accepted for publication in A&A

First results from high-resolution coronal spectroscopy with the {\it XMM-Newton} Reflection Grating Spectrometers (RGS) are reviewed. Five stellar systems (HR 1099, Capella, Procyon, YY Gem, AB Dor) have been observed. The emphasis of the present paper is on overall coronal structure. Elemental abundances in {\it active stars} are found to be `anomalous' in the sense that they tend to increase with increasing First Ionization Potential (FIP - i.e., signifying an inverse FIP effect). Coronal densities are measured at levels of a few times $10^{10}$ cm$^{-3}$ for cooler plasma, although there are indications for very high densities in the hotter plasma components., Comment: To appear in Proc. of "X-ray astronomy 2000",(Palermo Sep. 2000), Eds. R. Giacconi, L. Stella, S. Serio, ASP Conf. Series, in press

First results from high-resolution coronal spectroscopy of flares with the Reflection Grating Spectrometers on board the \textit{XMM-Newton} satellite are reviewed. Rotational modulation in the X-ray light curve of HR 1099 is discussed. Results from time-dependent spectroscopy of flares in the active stars HR 1099, AB Dor, YY Gem are also presented. Variations in the shape of the emission measure distributions, in the abundances and in the average density of the cool plasma are discussed., Comment: To appear in Proc. of "X-ray astronomy 2000",(Palermo Sep. 2000), Eds. R. Giacconi, L. Stella, S. Serio, ASP Conf. Series, in press

The RS CVn binary system HR 1099 was extensively observed by the XMM-Newton observatory in February 2000 as its first-light target. A total of 570 ks of exposure time was accumulated with the Reflection Grating Spectrometers (RGS). The integrated X-ray spectrum between 5-35 Angstrom is of unprecedented quality and shows numerous features attributed to transitions of the elements C, N, O, Ne, Mg, Si, S, Fe, and Ni. We perform an in-depth study of the elemental composition of the average corona of this system, and find that the elemental abundances strongly depend on the first ionisation potential (FIP) of the elements. But different from the solar coronal case, we find an inverse FIP effect, i.e., the abundances (relative to oxygen) increase with increasing FIP. Possible scenarios, e.g., selective enrichment due to Ne-rich flare-like events, are discussed., Comment: 5 pages, 4 figures. Accepted by A&A Letters, XMM issue

We present initial results from observations of the low-mass X-ray binary EXO 0748-67 with the Reflection Grating Spectrometer on board the XMM-Newton Observatory. The spectra exhibit discrete structure due to absorption and emission from ionized neon, oxygen, and nitrogen. We use the quantitative constraints imposed by the spectral features to develop an empirical model of the circumsource material. This consists of a thickened accretion disk with emission and absorption in the plasma orbiting high above the binary plane. This model presents challenges to current theories of accretion in X-ray binary systems., Comment: 5 pages, 4 figures, accepted by A&A letters, XMM special issue