  Highly precise temperatures (sigma = 10-15 K) have been determined from line
depth ratios for a set of 248 F-K field dwarfs of about solar metallicity (-0.5
< [Fe/H] < +0.4), based on high resolution (R=42000), high S/N echelle spectra.
A new gap has been discovered in the distribution of stars on the Main Sequence
in the temperature range 5560 to 5610 K. This gap coincides with a jump in the
microturbulent velocity Vt and the well-known Li depression near 5600 K in
field dwarfs and open clusters. As the principal cause of the observed
discontinuities in stellar properties we propose the penetration of the
convective zone into the inner layers of stars slightly less massive than the
Sun and related to it, a change in the temperature gradient.
