  We report optical and near-infrared observations of SN 2012ca with the Public
ESO Spectroscopy Survey of Transient Objects (PESSTO), spread over one year
since discovery. The supernova (SN) bears many similarities to SN 1997cy and to
other events classified as Type IIn but which have been suggested to have a
thermonuclear origin with narrow hydrogen lines produced when the ejecta impact
a hydrogen-rich circumstellar medium (CSM). Our analysis, especially in the
nebular phase, reveals the presence of oxygen, magnesium and carbon features.
This suggests a core collapse explanation for SN2012ca, in contrast to the
thermonuclear interpretation proposed for some members of this group. We
suggest that the data can be explained with a hydrogen and helium deficient SN
ejecta (Type I) interacting with a hydrogen-rich CSM, but that the explosion
was more likely a Type Ic core-collapse explosion than a Type Ia thermonuclear
one. This suggests two channels (both thermonuclear and stripped envelope
core-collapse) may be responsible for these SN 1997cy-like events.
