  We present the first (as of February, 1999) UBV and uvby photometric
measurements for the short period variable star V2109 Cyg discovered by the
Hipparcos 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 and
the most recent synthetic colour grids. The results are:
  [Fe/H]=-0.9+/-0.2, Mbol=0.73+/-0.43 mag, <Teff>=6800+/-200 K, log
g=2.7+/-0.2, R=4.6+/-0.9 Ro.
  Additionally, we took medium resolution (lambda/Delta lambda=11000) spectra
in the red spectral region centered at 6600 A. A complete radial velocity curve
has been determined from 60 spectra. We found no systematic difference between
the velocities from Halpha and metallic lines indicating a smooth pulsation.
The mass of the star is found to be M=0.5+/-0.3 Mo using the photometric log g
value and the acceleration curve calculated from the radial velocity data. The
finally adopted set of the physical parameters lies in the typical range of RR
Lyrae stars, which implies that V2109 Cyg is the shortest period RR Lyrae-type
variable. The visual amplitude vs. period and the Fourier amplitude parameter
R21} vs. period diagrams suggest that V2109 Cyg probably pulsates in the second
overtone mode.
