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There are very few confirmed black holes with a mass that could be $\sim\!4\, M_\odot$ and no neutron stars with masses greater than $\sim\! 2\,M_\odot$, creating a gap in the observed distribution of compact star masses.Some black holes with masses between 2 and $4\, M_\odot$ might be hiding amongother X-ray sources, whose masses are difficult to measure. We present newhigh-speed optical photometry of the low-mass X-ray binary V1408 Aql (= 4U1957+115), which is a persistent X-ray source thought to contain a black hole.The optical light curve of V1408~Aql shows a nearly sinusoidal modulation atthe orbital period of the system superimposed on large night-to-nightvariations in mean intensity. We combined the new photometry withpreviously-published photometry to derive a more precise orbital period, $P =0.388893(3)$\ d, and to better define the orbital light curve andnight-to-night variations. The orbital light curve agrees well with a model inwhich the modulation is caused entirely by the changing aspect of the heatedface of the secondary star. The lack of eclipses rules out orbital inclinationsgreater than $65^{\circ}$. Our best models for the orbital light curve favorinclinations near $13^{\circ}$ and black hole masses near $3\, M_\odot$ with a90\% upper bound of $6.2\, M_\odot$, and a lower bound of $2.0\, M_\odot$imposed solely by the maximum mass of neutron stars. We favor a black holeprimary over a neutron star primary based on evidence from the X-ray spectra,the high spin of the compact object, and the fact that a type I X-ray burst hasnot been observed for this system. Although uncertainties in the data and themodels allow higher masses, possibly much higher masses, the compact star inV1408~Aql is a viable candidate for a black hole lying in the mass gap.

The Case for a Low Mass Black Hole in the Low Mass X-ray Binary V1408 Aquilae (= 4U 1957+115)