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A growing number of eclipsing binary systems of the "HW Vir" kind (i. e.,composed by a subdwarf-B/O primary star and an M dwarf secondary) showvariations in their orbital period, also called Eclipse Time Variations (ETVs).Their physical origin is not yet known with certainty: while some ETVs havebeen claimed to arise from dynamical perturbations due to the presence ofcircumbinary planetary companions, other authors suggest that the Applegateeffect or other unknown stellar mechanisms could be responsible for them. Inthis work, we present twenty-eight unpublished high-precision light curves ofone of the most controversial of these systems, the prototype HW Virginis. Wehomogeneously analysed the new eclipse timings together with historical dataobtained between 1983 and 2012, demonstrating that the planetary modelspreviously claimed do not fit the new photometric data, besides beingdynamically unstable. In an effort to find a new model able to fit all theavailable data, we developed a new approach based on a global-search geneticalgorithm and eventually found two new distinct families of solutions that fitthe observed timings very well, yet dynamically unstable at the 10^5-year timescale. This serves as a cautionary tale on the existence of formal solutionsthat apparently explain ETVs but are not physically meaningful, and on the needof carefully testing their stability. On the other hand, our data confirm thepresence of an ETV on HW Vir that known stellar mechanisms are unable toexplain, pushing towards further observing and modelling efforts.

A new photometric and dynamical study of the eclipsing binary star HW Virginis