INDEPENDENT DISTANCE DETERMINATIONS TO MILKY-WAY CEPHEIDS IN OPEN CLUSTERS AND ASSOCIATIONS .1. THE BINARY CEPHEID DL CAS IN NGC 129

As an eight-day Cepheid which is both a component of a spectroscopic b inary and a member of the open cluster NGC 129, DL Cas is potentially a very accurate calibrator of the period-luminosity (PL) relation and Cepheid mass. From 160 high-precision (sigma < 1.5 km/s) radial veloci ty observations made with the CORAVEL and DAO spectrometers-including 67 new unpublished data-we have obtained both the orbital and pulsatio nal velocity curves of this binary Cepheid. This body of RV data makes DL Cas one of the best observed Cepheids in our galaxy. Our analysis yields an orbital period of 684.4 +/- 0.4 days which confirms DL Cas a s one of the shortest-period binaries containing a Cepheid. We derive new precise orbital elements which replace earlier preliminary values found by Harris et al. Isochrone fitting to the V,B - V data points of Turner et al. yields an age of NGC 129 of (7.6 +/- 0.4) X 10(7) yr an d a Cepheid mass of 5.6 M.. Evidence from age, a possible period chang e, and strip crossing times suggest that DL Cas is a solar-abundance s tar making its third (redward) crossing through the Cepheid instabilit y strip. Existing observational constraints from our mass function of the DL Cas system and an IUE spectrum suggest that the companion is a main sequence star in the mass range from 2.6 to 5.6 M.. We use the pu lsational velocity curve and published photometry to derive the distan ce and mean radius of DL Cas with the surface brightness method, findi ng values of 2034 +/- 110 pc and 66.0 +/- 3.5R., respectively. The rad ius we derive indicates that DL Cas is a fundamental-mode pulsator, re moving any possible ambiguity in mode identification. The distance cor responds to a mean absolute visual magnitude of [M(V)] = -4.2 +/- 0.3 mag whose error is dominated by the uncertainty of the absorption corr ection. Since our very precise distance contributes only 0.12 mag to t he error in (Mv), improved reddening studies of NGC 129 would make DL Cas a very tight calibrator of the PL relation. Our value of the gamma velocity of the DL Cas system is identical to the mean radial velocit y of the stars in NGC 129, strengthening the case for cluster membersh ip. However, our distance for DL Cas, and thus for NGC 129, is signifi cantly larger than the 1670 +/- 13 pc obtained by Turner et al. from Z AMS fitting of the cluster. Possible causes for this discrepancy, and their implications for Cepheid distance scale calibrations, are discus sed.