On the binary nature of the γ-ray sources AGL J2241+4454 (= MWC 656) and HESS J0632+057 (= MWC 148)

We present optical spectroscopy of MWC 656 and MWC 148, the proposed optical counterparts of the gamma-ray sources AGL J2241+4454 and HESS J0632+057, respectively. The main parameters of the H alpha emission line [equivalent width (EW), full width at half-maximum and centroid velocity] in these stars are modulated on the proposed orbital periods of 60.37 and 321 d, respectively. These modulations are likely produced by the resonant interaction of the Be discs with compact stars in eccentric orbits. We also present radial velocity curves of the optical stars folded on the above periods and obtain the first orbital elements of the two gamma-ray sources, thus confirming their binary nature. Our orbital solution supports eccentricities e similar to 0.4 and 0.83 +/- 0.08 for MWC 656 and MWC 148, respectively. Furthermore, our orbital elements imply that the X-ray outbursts in HESS J0632+057/MWC 148 are delayed similar to 0.3 orbital phases after periastron passage, similar to the case of LS I +61 303. In addition, the optical photometric light-curve maxima in AGL J2241+4454/MWC 656 occur similar to 0.25 phases passed periastron, similar to what is seen in LS I + 61 303. We also find that the orbital eccentricity is correlated with the orbital period for the known gamma-ray binaries. This is explained by the fact that small stellar separations are required for the efficient triggering of very high energy radiation. Another correlation between the EW of H alpha and orbital period is also observed, similar to the case of Be/X-ray binaries. These correlations are useful to provide estimates of the key orbital parameters P-orb and e from the H alpha line in future Be gamma-ray binary candidates.