INDEPENDENT DISTANCE DETERMINATIONS TO MILKY-WAY CEPHEIDS IN OPEN CLUSTERS AND ASSOCIATIONS .2. CF CAS IN NGC-7790

Containing three Cepheids and an eclipsing binary, the open cluster NG C 7790 has the potential to be a Rosetta Stone for refining the zero p oint of the extragalactic distance scale. Unfortunately, ZAMS fitting of this relatively sparse, heavily reddened cluster is fraught with di fficulty, and even modern determinations of its distance modulus based on comparable data and techniques differ by over 0.3 mag. To provide an independent calibration of the distance to NGC 7790, we have perfor med a surface brightness analysis of the Cepheid CF Cas (P similar or equal to 4.88 days), which is widely believed to be a member of the cl uster. For this analysis, we use new high-quality radial-velocity data obtained with the DAO Radial Velocity Spectrometer, as well as archiv al velocities and photometry. Our new measurements increase the availa ble set of published RV data for CF Cas by over a third. We derive the distance and radius of CF Cas to be 3130 +/- 160 pc and 40.6 +/- 2.0R ., respectively. This radius is consistent with fundamental-mode pulsa tion for a Cepheid of this period, removing a possible ambiguity in mo de identification which could bias a period-luminosity calibration inc orporating CF Gas. We go on to estimate the absolute magnitude ([M(V)] similar or equal to -3.05 +/- 0.10 mag) and effective temperature (lo g T-eff similar or equal to 3.74 +/- 0.02) of CF Gas, although we caut ion that these values are sensitive to the assumed reddening (unlike o ur surface-brightness distance and radius). Evolutionary and Wesselink relations yield very similar values for the mass of CF Gas, near 5 M. . (C) 1995 American Astronomical Society.