Follow-up Opt. and NIR obs. of the SNIa SN2015bp

We present comprehensive optical and near-infrared (NIR) data on the "transitional" Type Ia SN 2015bp. An early NIR spectrum (t=-9.9 days with respect to B-band maximum) displays a striking CI 1.0693{mu}m line at 11.9x10^3^km/s, distinct from the prominent Mg ii 1.0927 {mu}m feature, which weakens toward maximum light.

SN 2015bp also displays a clear CII 6580{AA} notch early (t=-10.9 days) at 13.2x10^3^km/s, consistent with our NIR carbon detection. At M_B_=-18.46, SN 2015bp is less luminous than a normal SNIa and, along with iPTF 13ebh, is the second member of the transitional subclass to display prominent early-time NIR carbon absorption. We find it unlikely that the CI feature is misidentified HeI 1.0830{mu}m because this feature grows weaker toward maximum light, while the helium line produced in some double-detonation models grows stronger at these times.