Context :

Aims : We study PTF11mnb , a He-poor supernova ( SN ) whose light curves resemble those of SN 2005bf , a peculiar double-peaked stripped-envelope ( SE ) SN , until the declining phase after the main peak .
We investigate the mechanism powering its light curve and the nature of its progenitor star .

Methods : Optical photometry and spectroscopy of PTF11mnb are presented .
Light curves , colors and spectral properties are compared to those of SN 2005bf and normal SE SNe .
A bolometric light curve is built and modeled with the SNEC hydrodynamical code explosion of a MESA progenitor star , as well as with semi-analytic models .

Results : The light curve of PTF11mnb turns out to be similar to that of SN 2005bf until \sim 50 d , when the main ( secondary ) peaks occur at -18.5 mag .
The early peak occurs at \sim 20 d , and is about 1.0 mag fainter .
After the main peak , the decline rate of PTF11mnb is remarkably slower than what was observed in SN 2005bf , and it traces well the ^ { 56 } Co decay rate .
The spectra of PTF11mnb reveal a SN Ic , with no traces of He unlike in the case of SN Ib 2005bf , although with velocities comparable to those of SN 2005bf .
The whole evolution of the bolometric light curve is well reproduced by the explosion of a massive ( M _ { ej } \leavevmode \nobreak = 7.8 M _ { \odot } ) , He-poor star characterized by a double-peaked ^ { 56 } Ni distribution , a total ^ { 56 } Ni mass of 0.59 M _ { \odot } and an explosion energy of 2.2 \times 10 ^ { 51 } erg .
Alternatively , a normal SN Ib/c explosion ( M ( ^ { 56 } Ni ) = 0.11 M _ { \odot } , E _ { K } = 0.2 \times 10 ^ { 51 } erg , M _ { ej } \leavevmode \nobreak = 1 M _ { \odot } ) can power the first peak while a magnetar ( with a magnetic field characterized by B =5.0 \times 10 ^ { 14 } G , and a rotation period of P = 18.1 ms ) provides energy for the main peak .
The early g -band light curve can be fit with a shock-breakout cooling-tail or an extended envelope model , from which a radius of at least 30 R _ { \odot } is obtained .

Conclusions : We presented a scenario where PTF11mnb was the explosion of a massive , He-poor star , characterized by a double-peaked ^ { 56 } Ni distribution .
In this case , the ejecta mass and the absence of He imply a large ZAMS mass ( \sim 85 \leavevmode \nobreak M _ { \odot } ) for the progenitor , which most likely was a Wolf-Rayet star , surrounded by an extended envelope formed either by a pre-SN eruption or due to a binary configuration .
Alternatively , PTF11mnb could be powered by a SE SN with a less massive progenitor during the first peak and by a magnetar afterwards .