combine month and day into one date column

Using R, I'd like to combine into one column (date) the month nb (month) and the day nb (day) contained in two different columns and use the created column in a date format.

my data frame looks like this:

St Dep Month Day
A 2 1 1
B 2 1 1
B 2 2 1
A 5 1 1
A 7 1 1
C 2 1 1
C 5 1 1

And I want to add a column with something like "Jan-01".