CCD photometry of galactic open clusters – I. NGC 7790

A new BVRI CCD-photometry of the cluster NGC 7790 is presented. Of the 343 stars measured, 153 are considered to be cluster members. Assuming R = 3.06 at (B– V) = 0.0 and the observed Zero Age Main Sequence of the Pleiades as reference locus with distance modulus 5.57 (Walker 1985), we have derived from the resulting colour-magnitude diagrams new and more accurate estimates of the reddening and distance modulus of the cluster which turn out to be |$E(B-V) = 0.54 \pm 0.04 \,\text{and}\,{(m- M)}_{0}=12.65 \pm 0.15$|⁠, respectively. With the parameters obtained and assuming solar compositions, the age of NGC 7790 is |$(5.0 \pm 1.5) \times {10}^{7}$| yr using ‘standard’ models or |$(1.0 \pm 0.2) \times {10}^{8} \text{yr}$| adopting models with overshooting. This latter estimate seems to be more in agreement with the ages obtained by applying the period–age relations to the three Cepheids commonly assumed to be cluster members. Finally, the three Cepheids have been used to compute the zero-points of the period–luminosity and period–luminosity–colour relations based on these three stars.