THE ASTRONOMICAL JOURNAL    VOLUME 107, NUMBER 6, PAGE 2093           JUNE 1994

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

                              WOLFGANG P. GIEREN

  Astrophysics Group, Facultad de Fisica, Pontificia Universidad Catolica de
                    Chile, Casilla 104, Santiago 22, Chile
                     Electronic mail: wgieren@astro.puc.cl

                               DOUGLAS L. WELCH

           Department of Physics and Astronomy McMaster University,
                       Hamilton, Ontario L8S 4M1, Canada
               Electronic mail: welch@physun.physics.mcmaster.ca

                            JEAN-CLAUDE MERMILLIOD

          Institut d'Astronomie de l'Universite de Lausanne, CH-1290
                        Chavannes-des-Bois, Switzerland
                    Electronic mail: mermio@scsun.unige.ch

                              JAYMIE M. MATTHEWS

    Department of Geophysics and Astronomy, University of British Columbia,
               129-2219 Main Mall, Vancouver, BC, V6T 2A6, Canada
                    Electronic mail: matthews@astro.ubc.ca

                                GISELA HERTLING

  Astrophysics Group, Facultad de Fisica, Pontificia Universidad Catolica de
                    Chile, Casilla 104, Santiago 22, Chile
                      Electronic mail: ghert@astro.puc.cl


                                   ABSTRACT
    
    As an eight-day Cepheid which is both a component of a spectroscopic
    binary 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
    velocity observations made with the CORAVEL and DAO spectrometers --
    including 67 new unpublished data -- we have obtained both the orbital
    and pulsational 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 as 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 and a Cepheid mass of 5.6 Msun.  Evidence from age, a
    possible period change, and strip crossing times suggest that DL Cas is
    a solar-abundance star making its third (redward) crossing through the
    Cepheid instability 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
    Msun.  We use the pulsational velocity curve and published photometry
    to derive the distance and mean radius of DL Cas with the surface
    brightness method, finding values of 2034 +/- 110 pc and 66.0 +/- 3.5
    Rsun, respectively.  The radius we derive indicates that DL Cas is a
    fundamental-mode pulsator, removing any possible ambiguity in mode
    identification.  The distance corresponds to a mean absolute visual
    magnitude of <Mv> = -4.2 +/- 0.3 mag whose error is dominated by the
    uncertainty of the absorption correction.  Since our very precise
    distance contributes only 0.12 mag to the 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 velocity of the stars in NGC
    129, strengthening the case for cluster membership.  However, our
    distance for DL Cas, and thus for NGC 129, is significantly larger than
    the 1670 +/- 13 pc obtained by Turner et al. from ZAMS fitting of the
    cluster.  Possible causes for this discrepancy, and their implications
    for Cepheid distance scale calibrations, are discussed.