Optical and Infrared Spectroscopy of SN 1999ee and SN 1999ex

We report optical and infrared spectroscopic observations of the Type Ia SN 1999ee and the Type Ib/c SN 1999ex, both of which were hosted by the galaxy IC 5179. For SN 1999ee we obtained a continuous sequence with an unprecedented wavelength and temporal coverage beginning 9 days before maximum light and extending through day 42. Before maximum light SN 1999ee displayed a normal spectrum with a strong Si II λ6355 absorption, thus showing that not all slow-declining supernovae (SNe) are spectroscopically peculiar at these evolutionary phases. A comparative study of the infrared spectra of SN 1999ee and other Type Ia SNe shows that there is a remarkable homogeneity among the Branch-normal SNe Ia during their first 60 days of evolution. SN 1991bg-like objects, on the other hand, display spectroscopic peculiarities at infrared wavelengths. SN 1999ex was characterized by the lack of hydrogen lines, weak optical He I lines, and strong He I λλ10830, 20581, thus providing an example of an intermediate case between pure Ib and Ic supernovae. We conclude, therefore, that SN 1999ex provides the first clear evidence for a link between the Ib and Ic classes and that there is a continuous spectroscopic sequence ranging from the He-deficient SNe Ic to the SNe Ib, which are characterized by strong optical He I lines. Based on observations collected at the European Southern Observatory, Chile (program ESO 164.H-0376).