The star Sirius, the brightest star in our northern skies, has a white-dwarf companion.

MSun

Remember that a white dwarf can't have a mass greater than 1.4 MSun

Assuming that the two stars formed at the same time, discuss with your group how Sirius could have a white-dwarf companion. 

Hint: Was the initial mass of the white-dwarf star larger or smaller than that of Sirius?