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About one third of the 3033 $\gamma$-ray sources in the Third Fermi-LATGamma-ray Source Catalogue (3FGL) are unidentified and do not have even atentative association with a known object, hence they are defined asunassociated. Among Galactic $\gamma$-ray sources, pulsars represent thelargest class, with over 200 identifications to date. About one third of themare milli-second pulsars (MSPs) in binary systems. Therefore, it is plausiblethat a sizeable fraction of the unassociated Galactic $\gamma$-ray sourcesbelong to this class. We collected X-ray and optical observations of the fieldsof twelve unassociated Fermi sources that have been classified as likely MSPsaccording to statistical classification techniques. To find observationalsupport for the proposed classification, we looked for periodic modulations ofthe X-ray and optical flux of these sources, which could be associated with theorbital period of a MSP in a tight binary system. Four of the observed sourceswere identified as binary MSPs, or proposed as high-confidence candidates,while this work was in progress. For these sources, we present the results ofour follow-up investigations, whereas for the others we present possibleevidence of new MSP identifications. In particular, we discuss the case of 3FGLJ0744.1-2523 that we proposed as a possible binary MSP based upon thepreliminary detection of a 0.115 d periodicity in the flux of its candidateoptical counterpart. We also found very marginal evidence of periodicity in thecandidate optical counterpart to 3FGL J0802.3-5610, at a period of 0.4159 d,which needs to be confirmed by further observations.

A multi-wavelength investigation of candidate milli-second pulsars in unassociated $\gamma$-ray sources