Determination of masses has been a long standing problem in the Cepheid research. Since the early days of Cepheid modeling different methods of mass calibration have lead to conflicting results, implying serious discrepancies between the evolution and pulsation theories (see Cox 1980 for a review). In recent years this situtation has been mostly remedied, and the Baade-Wesselink masses, pulsation masses and evolutionary masses are now in good agreement with each other (e.g., Gieren 1989). However, both the bump masses inferred from the position of the secondary bump on the light curve and the beat masses obtained from the period ratios of the double mode Cepheids turned out to be very resilient to a reconciliation.

There are 13 Cepheids in the Galaxy in which two vibrational modes are simultaneously excited (e.g., Szabados 1988). The period ratios measured in these variables can be used in conjunction with the linear pulsation theory to infer the masses of these stars. The method was first applied by Petersen (1973) who obtained masses ranging from 1Mʘ to 3Mʘ for Cepheids with fundamental mode periods between 2.1 d and 6.3 d. Such ‘beat” masses are 2—4 times smaller than the evolutionary or Baade-Wesselink masses for these objects.