THE TRIPLE-MODE VARIABLE AC ANDROMEDAE

We have calculated a large number of linear radial pulsation models to fit the periods of AC And. In the parameter regime representative of RR Lyrae stars no models can be found that match the observed periods for any heavy element abundance Z. There are, however, two groups of h igher mass models which yield complete agreement between the observed periods and those of the first three radial modes. The first comprises models with 0.001 < Z < 0.002 and masses and luminosities of 1.1 < M/ M. < 1.4, 100 < L/L. < 130 (depending on Z). The second group has high er masses, luminosities and Z, viz. 0.01 < Z < 0.02, M/M. almost-equal -to 2.7, 80 < L/L. < 200. These ranges are sensitive to the assumed un certainties of the model periods. We give ranges of model parameters a s functions of Z and discuss their uncertainties. Present observationa l constraints and the limited sensitivity of the method of fitting per iods do not allow us to sufficiently narrow down the derived parameter ranges. Furthermore, although recent evolution calculations are in fa ir agreement with both low- and high-mass models, the latter ones are preferred because the theoretical period changes are in better agreeme nt with the observations. Therefore, we suggest that AC And is an evol ved high-mass star, with similarities to the delta Scuti type stars, o r perhaps a rare oddball. As a side product of our survey, we give acc urate fitting formulae for the periods and period ratios.