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We have used NIRSPEC on Keck II to obtain $K$-band spectroscopy of the lowmass X-ray binary V616 Mon (= A0620$-$00). V616 Mon is the proto-typical softx-ray transient containing a black hole primary. As such it is important toconstrain the masses of the binary components. The modeling of the infraredobservations of ellipsoidal variations in this system lead to a derived mass of11.0 M$_{\sun}$ for the black hole. The validity of this derivation has beencalled into question due to the possiblity that the secondary star's spectralenergy distribution is contaminated by accretion disk emission (acting todilute the variations). Our new $K$-band spectrum of V616 Mon reveals alate-type K dwarf secondary star, but one that has very weak $^{\rm 12}$COabsorption features. Comparison of V616 Mon with SS Cyg leads us to estimatethat the accretion disk supplies only a small amount of $K$-band flux, and theellipsoidal variations are not seriously contaminated. If true, the derivedorbital inclination of V616 Mon is not greatly altered, and the mass of theblack hole remains large. A preliminary stellar atmosphere model for the$K$-band spectrum of V616 Mon reveals that the carbon abundance isapproximately 50% of the solar value. We conclude that the secondary star inV616 Mon has either suffered serious contamination from the accretion ofsupernova ejecta that created the black hole primary, or it is the strippedremains of a formerly more massive secondary star, one in which the CNO cyclehad been active.Comment: 20 pages, 5 figure

The Nature of the Secondary Star in the Black Hole X-Ray Transient V616 Mon (=A0620-00)