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We present 39 nights of optical photometry, 34 nights of infrared photometry,and 4 nights of optical spectroscopy of the Type Ia SN 1999ac. This supernovawas discovered two weeks before maximum light, and observations were begunshortly thereafter. At early times its spectra resembled the unusual SN 1999aaand were characterized by very high velocities in the Ca II H and K lines, butvery low velocities in the Si II 6355 A line. The optical photometry showed aslow rise to peak brightness but, quite peculiarly, was followed by a morerapid decline from maximum. Thus, the B- and V-band light curves cannot becharacterized by a single stretch factor. We argue that the best measure of thenature of this object is not the decline rate parameter Delta m_15 (B). The B-Vcolors were unusual from 30 to 90 days after maximum light in that they evolvedto bluer values at a much slower rate than normal Type Ia supernovae. Thespectra and bolometric light curve indicate that this event was similar to thespectroscopically peculiar slow decliner SN 1999aa.Comment: 42 pages, 14 figures, accepted for publication in the Astronomical  Journal (January 28, 2006

Optical and Near-Infrared Observations of the Peculiar Type ia Supernova 1999ac