The problem of non-constant periodicities or multi-periodic phenomena is often encountered in Astrophysics. These phenomena are usually interpreted either with Fourier transforms or with overlapping functions, or even using a combination of them. In this work, we present a new method based on the idea of a variable sine function. We apply it to data analysis of actual binary stars and compare the results. Moreover, with its aid we describe the subsurface field of active binaries exhibiting magnetic activity cycles. The method can also be implemented in various other topics of applied sciences.