Observations of the binary microlens event MACHO 98-SMC-1 by the microlensing planet search collaboration

We present observations of the binary lensing event MACHO 98-SMC-1 conducte d at the Mount Stromlo 1.9 m telescope by the Microlensing Planet Search (M PS) collaboration. The MPS data constrain the first caustic crossing to hav e occurred after 1998 June 5.55 UT and thus directly rule out one of the tw o fits presented by the PLANET collaboration (model II). This substantially reduces the uncertainty in the relative proper-motion estimations of the l ens object. We perform joint binary microlensing fits of the MPS data toget her with the publicly available data from the EROS, MACHO/GMAN, and OGLE co llaborations. We also study the binary lens fit parameters previously publi shed by the PLANET and MACHO/GMAN collaborations by using them as initial v alues for chi(2)-minimization. Fits based on the PLANET model I appear to b e in conflict with the GMAN-CTIO data. From our best fit, we find that the lens system has a proper motion of mu = 1.5 +/- 0.3 km s(-1) kpc(-1) with r espect to the source, which implies that the lens system is most likely to be located in the Small Magellanic Cloud, strengthening the conclusion of p revious reports.