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We propose a class of new cutting planes to strengthen the Lov\'asz theta function, a well-known semidefinite programming (SDP) relaxation for the stable set and the graph coloring problems. For both problems, we introduce two new cutting planes that are derived from odd-cycle constraints and triangle inequalities and are valid for certain subgraphs induced by odd cycles. Computational experiments on graphs from the literature show that for each problem, one of the proposed cutting planes gives very good bounds.

We consider semidefinite relaxations of Stable-Set and Coloring, which are based on quadratic 0-1 optimization. Information about the stability number and the chromatic number is hidden in the objective function. This leads to simplified relaxations which depend mostly on the number of vertices of the graph. We also propose tightenings of the relaxations which are based on the maximal cliques of the underlying graph. Computational results on graphs from the literature show the strong potential of this new approach.