  M-dwarf stars provide very favourable conditions to find habitable worlds
beyond our solar system. The estimation of the fundamental parameters of the
transiting exoplanets rely on the accuracy of the theoretical predictions for
radius and effective temperature of the host M-dwarf, hence the importance of
multiple empirical tests of very low-mass star (VLM) models, the theoretical
counterpart of M-dwarfs. Recent determinations of mass, radius and effective
temperature of a sample of M-dwarfs of known metallicity have disclosed a
supposed discontinuity in the effective temperature-radius diagram
corresponding to a stellar mass of about 0.2Mo, that has been ascribed to the
transition from partially convective to fully convective stars. In this paper
we compare existing VLM models to these observations, and find that theory does
not predict any discontinuity at around 0.2Mo, rather a smooth change of slope
of the effective temperature-radius relationship around this mass value. The
appearance of a discontinuity 5is due to naively fitting the empirical data
with linear segments. Also, its origin is unrelated to the transition to fully
convective structures. We find that this feature is instead an empirical
signature for the transition to a regime where electron degeneracy provides an
important contribution to the stellar EOS, and constitutes an additional test
of the consistency of the theoretical framework for VLM models.
