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The first BV bands photometric observations and the low-resolution spectrumof the shortest period Am type eclipsing binary TYC 6408-989-1 have beenobtained. The stellar atmospheric parameters of the primary star were obtainedthrough the spectral fitting as follows: $T_{eff}=6990\pm117 K$, $\logg=4.25\pm0.26 cm/s^2$, $[Fe/H]=-0.45\pm0.03 dex$. The original spectra obtainedby 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 amarginal Am (Am:) star with a spectral type of kA3hF1mA5 IV-V identifiedthrough the MKCLASS program. The observed light curves were analyzed throughthe Wilson-Devinney code. The final photometric solutions show that TYC6408-989-1 is a marginal contact binary with a low mass ratio (q=0.27). Thetemperature of the secondary component derived through the light curve analysisis significantly higher than main sequence stars. In addition, TYC 6408-989-1is a poor thermal contact binary. The temperature differences between the twocomponents is about 1800K. TYC 6408-989-1 should be located in the oscillationstage predicted by the thermal relaxation oscillations theory (TRO) and willevolve into the shallow contact stage eventually. The very short period (lessthan 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 thecut-off of rotation velocities and periods of Am stars. We have collected thewell known eclipsing Am binaries with absolute parameters from the literature.

Investigation of the shortest period Am type eclipsing binary TYC 6408-989-1