1SWASP J162117.36 + 441254.2 was originally classified as an EW-type binary with a period of 0.20785 days .
However , it was detected to have undergone a stellar outburst on June 3 , 2016 .
Although the system was latter classified as a cataclysmic variable ( CV ) and the event was attributed as a dwarf-nova outburst , the physical reason is still unknown .
This binary has been monitored photometrically since April 19 , 2016 and many light curves were obtained before , during and after the outburst .
Those light and color curves observed before the outburst indicate that the system is a special CV .
The white dwarf is not accreting material from the secondary and there are no accretion disks surrounding the white dwarf .
By comparing the light curves obtained from April 19 to September 14 , 2016 , it was found that magnetic activity of the secondary is associated with the outburst .
We show strong evidence that the L _ { 1 } region on the secondary was heavily spotted before and after the outburst and thus quench the mass transfer , while the outburst is produced by a sudden mass accretion of the white dwarf .
These results suggest that J162117 is a good astrophysical laboratory to study stellar magnetic activity and its influences on CV mass transfer and mass accretion .