Investigation of the shortest period Am type eclipsing binary TYC
  6408-989-1
  The first BV bands photometric observations and the low-resolution spectrum
of the shortest period Am type eclipsing binary TYC 6408-989-1 have been
obtained. The stellar atmospheric parameters of the primary star were obtained
through the spectral fitting as follows: $T_{eff}=6990\pm117 K$, $\log
g=4.25\pm0.26 cm/s^2$, $[Fe/H]=-0.45\pm0.03 dex$. The original spectra obtained
by European Southern Observatory (ESO) were processed with IRAF package by us.
Based on the ESO blue-violet spectra, TYC 6408-989-1 was concluded as a
marginal Am (Am:) star with a spectral type of kA3hF1mA5 IV-V identified
through the MKCLASS program. The observed light curves were analyzed through
the Wilson-Devinney code. The final photometric solutions show that TYC
6408-989-1 is a marginal contact binary with a low mass ratio (q=0.27). The
temperature of the secondary component derived through the light curve analysis
is significantly higher than main sequence stars. In addition, TYC 6408-989-1
is a poor thermal contact binary. The temperature differences between the two
components is about 1800K. TYC 6408-989-1 should be located in the oscillation
stage predicted by the thermal relaxation oscillations theory (TRO) and will
evolve into the shallow contact stage eventually. The very short period (less
than one day), marginal Am peculiarity and quit large rotational velocity
($v\sin i \simeq 160 km s^{-1}$) make TYC 6408-989-1 became a challenge to the
cut-off of rotation velocities and periods of Am stars. We have collected the
well known eclipsing Am binaries with absolute parameters from the literature.
