V2109 Cygni, a second overtone field RR Lyrae star

We present the first (as of February, 1999) UBV and uvby photometric measur ements for the short period variable star V2109 Cyg discovered by the Hippa rcos satellite and classified as a field RRc variable. We have obtained new times of maxima and the period change has been studied. We determined the fundamental physical parameters using the geometric distance of the star an d the most recent synthetic colour grids. The results are: [Fe/H] = -0.9 +/- 0.2, M-bol = 0.73 +/- 0.43 mag, [T-eff] = 6800 +/- 200 K, logg = 2.7 +/- 0.2, R = 4.6 +/- 0.9 R.. Additionally, we took medium resolution (lambda/Delta lambda approximate to 11000) spectra in the red spectral region centered at 6600 Angstrom. A com plete radial velocity curve has been determined from 60 spectra. We found n o systematic difference between the velocities from Her and metallic lines indicating a smooth pulsation. The mass of the star is found to be M = 0.5 +/- 0.3 M. using the photometric logg value and the acceleration curve calc ulated from the radial velocity data. The finally adopted set of the physic al parameters lies in the typical range of RR Lyrae stars, which implies th at V2109 Cyg is the shortest period RR Lyrae-type variable. The visual ampl itude vs. period and the Fourier amplitude parameter R-21 vs. period diagra ms suggest that V2109 Cyg probably pulsates in the second overtone mode.