[en] Observations of the light variations of AC Andromedae and TU Cassiopeiae are reviewed. The radial fundamental, first overtone, second overtone, and three nonlinear coupling beat pulsation periods of AC And are presented, confirming the Fitch and Sxeidl work. For TU Cas the radial fundamental, first overtone, and several nonlinear coupling beat periods can be found, and a Fourier analysis has confirmed Faulkner's result at 1.25 days; the small amplitude of this third periodicity, however, makes it reality uncertain. Using an inhomogeneous model with helium enrichment (Y=0.48) in the surface convection zones and mixed downward to the 300,000 K level or 5 x 10^-4 of the mass, the three periods give a mass for AC And as 3 M/sub sun/. This is consistent with an evolutionary first crossing of the Cepheid instability strip. TU Cas has periods that indicate a mass of 4 M/sub sun/ using a similar model but normal composition (Y=d0.356,Z=0.044) at the surface and enriched between 80,000 and 150,000 K (2 x 10^-4 of the mass) to Y=0.70. A first or second crossing is possible; but if the enrichment is by a Cepheid wind and downward mixing, there is more time for TU Cas on the second crossing. If the second overtone of TU Cas does not exist, the variable does not need the thin normal helium composition surface layer and is just another double-mode Cepheid with Yapprox.0.5 down to the 200,000 K level