Important progress has been made within the past few years regarding the theory of low mass stars (m < 1M⊙) and brown dwarfs. The main improvements concern the equation of state of dense plasmas and the modelling of cool and dense atmospheres, necessary for a correct description of such objects. These theoretical efforts now yield a better understanding of these objects and good agreement with observations regarding color-magnitude diagrams of globular clusters, mass-magnitude relationships and near-IR color-magnitude diagrams for young open clusters. However uncertainties still remain regarding synthetic optical colors and the complex problem of dust formation in the coolest atmosphere models.

Improvement of the theory

Very low mass (VLM) stars and brown dwarfs (BD) are dense and cool objects, with typical central densities of the order of 100−1000 gr.cm−3 and central temperatures lower than 107 K. Under such conditions, a correct equation of state (EOS) for the description of their inner structure must take into account strong correlations between particles, resulting in important departures from a perfect gas EOS (cf. Chabrier & Baraffe (1997)). Important progress has been made in this field, in particular by Saumon, Chabrier & Van Horn (1995) who developed an EOS specially devoted to VLM stars, BD and giant planets. Since the EOS determines essentially the mechanical structure of these objects, and thus the mass-radius relationship, it can be tested against observations of eclipsing binary systems.