Pandas : Get a column value where another column is the minimum in a sub-grouping

Question

Suppose I have :

SM  Week  Capacity
A    0     1000
A    1     110
B    1      980
B    2      89
B    3      89

And I want to get :

SM  Week  Capacity
A    0     1000
B    1     980

In other words, I want to get only the rows where the week is the minimum in the group defined by SM. How can I do this in pandas, outside of iterating row by row to check manually?

Thanks.