The galactic double-mode Cepheids II. Properties of the generalized   phase differences
  By considering the least-squares fits of the double-mode Cepheid light curves discussed in Paper I we defined their properties by their Fourier parameters and generalized phase differences $G_{i,j}$. When plotting the latter quantities as a function of the respective order, the second order terms are confined in the region just below 3/2$\pi$; the third order terms have $\pi/2 < G_{i,j} < \pi$, the fourth order ones cluster around 2$\pi$ (or 0), the fifth order ones seem to have $\pi < G_{i,j} < 3/2 \pi$. The mean $G_{i,j}$ values are also regularly spaced. The progression of the $G_{i,j}$ values as a function of the period was investigated and the signature of a possible resonance near 6.0 d was found. 
Ennio Poretti and Ilaria Pardo (Osservatorio Astronomico di Brera,   Merate, Italy)