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Early-time light curves of Type Ib/c supernovae from the SDSS-II Supernova Survey

We analyse the early-time optical light curves (LCs) of 20 Type Ib/c supernovae (SNe Ib/c) from the Sloan Digital Sky Survey (SDSS) SN survey II, aiming to study their properties as well as to derive their progenitor parameters. High-cadence, multi-band LCs are fitted with a functional model and the best-fit parameters are compared among the SN types. Bolometric LCs (BLCs) are constructed for the entire sample. We computed the black-body (BB) temperature (T_BB) and photospheric radius (R_ph) evolution for each SN via BB fits on the spectral energy distributions. In addition, the BLC properties are compared to model expectations. Complementing our sample with literature data, we find that SNe Ic and Ic-BL (broad-line) have shorter rise times than those of SNe Ib and IIb. Δm_15 is similar among the different sub-types. SNe Ic appear brighter and bluer than SNe Ib, but this difference vanishes if we consider host galaxy extinction corrections based on colors. Our SNe have typical T_BB ~ 10000 K at peak, and R_ph ~ 10^15 cm. Analysis of the BLCs of SNe Ib and Ic gives typical ejecta masses M_ej = 3.6-5.7 M_sun, energies E_K = 1.5-1.7x10^51 erg, and M(^56Ni) = 0.3 M_sun. Higher values for E_K and M(^56Ni) are estimated for SNe Ic-BL (M_ej = 5.4 M_sun, E_K = 10.7x10^51 erg, M(^56Ni) = 1.1 M_sun). For the majority of SNe Ic and Ic-BL we can put strong limits (<2-4 days) on the duration of the expected early-time plateau. In the case of SN Ib 2006lc, a >5.9 days plateau seems to be detected. The rising part of the BLCs is reproduced by power laws with index <2. For two SNe we a possible shock break-out cooling tail. Based on the limits for the plateau length and on the slow rise of the BLCs, we find that in most of our SNe Ic and Ic-BL the ^56Ni is mixed up to the outer layers, suggesting that SN Ic progenitors are de facto helium poor.