Improving the surface brightness-color relation for early-type stars
  using optical interferometry
  The aim of this work is to improve the SBC relation for early-type stars in
the $-1 \leq V-K \leq 0$ color domain, using optical interferometry.
Observations of eight B- and A-type stars were secured with the VEGA/CHARA
instrument in the visible. The derived uniform disk angular diameters were
converted into limb darkened angular diameters and included in a larger sample
of 24 stars, already observed by interferometry, in order to derive a revised
empirical relation for O, B, A spectral type stars with a V-K color index
ranging from -1 to 0. We also took the opportunity to check the consistency of
the SBC relation up to $V-K \simeq 4$ using 100 additional measurements. We
determined the uniform disk angular diameter for the eight following stars:
$\gamma$ Ori, $\zeta$ Per, $8$ Cyg, $\iota$ Her, $\lambda$ Aql, $\zeta$ Peg,
$\gamma$ Lyr, and $\delta$ Cyg with V-K color ranging from -0.70 to 0.02 and
typical precision of about $1.5\%$. Using our total sample of 132 stars with
$V-K$ colors index ranging from about $-1$ to $4$, we provide a revised SBC
relation. For late-type stars ($0 \leq V-K \leq 4$), the results are consistent
with previous studies. For early-type stars ($-1 \leq V-K \leq 0$), our new
VEGA/CHARA measurements combined with a careful selection of the stars
(rejecting stars with environment or stars with a strong variability), allows
us to reach an unprecedented precision of about 0.16 magnitude or $\simeq 7\%$
in terms of angular diameter.
