  We present photometric and spectroscopic observations of supernova (SN)
2002cx, which reveal it to be unique among all observed type Ia supernovae (SNe
Ia). SN 2002cx exhibits a SN 1991T-like premaximum spectrum, a SN 1991bg-like
luminosity, and expansion velocities roughly half those of normal SNe Ia.
Photometrically, SN 2002cx has a broad peak in the $R$ band and a plateau phase
in the $I$ band, and slow late-time decline. The $(B - V)$ color evolution is
nearly normal, but the $(V - R)$ and $(V - I)$ colors are very red. Early-time
spectra of SN 2002cx evolve very quickly and are dominated by lines from
Fe-group elements; features from intermediate-mass elements (Ca, S, Si) are
weak or absent. Mysterious emission lines are observed around 7000 \AA\ at
about 3 weeks after maximum brightness. The nebular spectrum of SN 2002cx is
also unique, consisting of narrow iron and cobalt lines. The observations of SN
2002cx are inconsistent with the observed spectral/photometric sequence, and
provide a major challenge to our understanding of SNe Ia. No existing
theoretical model can successfully explain all observed aspects of SN 2002cx.

Astrophysics