UBVI CCD Photometry of NGC 7790

UBVI CCD photometry of the intermediate age open cluster NGC 7790 has been obtained using AZT-22 1.5 m telescope (f/7.74) at the Maidanak Astronomical Observatory in Uzbekistan. NGC 7790 contains three delta Cep variable stars including CEa Cas, CEb Cas, and CF Cas. PSF photometry was carried out using IRAF/DAOPHOT for all observations. The total number of stars observed both in V and I filter was 1008 and the limiting magnitude was V approximate to 22. To determine atmospheric extinction coefficients and photometric zero points, many blue and red standard stars as well as the standard stars in the celestial equator under various airmass were observed. Photometric data were transformed into the standard Johnson-Cousins' UBVI standard system. From the analysis of UBVI color-magnitude diagram and color-color diagram, the color excess in V and I filter [E(B-V)=0.58 +/- 0.02], the selective extinction ratio in V and I filter [R-v=A(v)/E(B-V) =3.02 +/- 0.09] and distance modulus (V-o-M-v =12.65 +/- 0.10) of the cluster were determined. The age of the cluster was estimated to be log age=8.05 +/- 0.05 [yr] based on the position of these three Cepheid variables in the color-magnitude diagram, the isochrone of the Geneva group (Ekstrom et al., 2012-Z=0.019), and the isochrone of the Padova group (Bressan et al., 2012-Z=0.014) were used to compare each other. Of them, the Geneva models that considered stellar rotation well described the position of delta Cepheid variables in the blue loop. Although they were well consistent with standard period-luminosity relation of delta Cepheid variables, three Cepheid variables in NGC 7790 were, on average, brighter by about 0.5 mag than the absolute magnitude estimated from the mean period-luminosity relation at a given period.