In Competition

After we have overcalled, partner shows his suit with a transfer bid and opps double

example: 

(1♣) - 1♥ - (pass) - 2♣

(dbl) 

pass    =    0-1 ♦

rdbl    =   2 ♦

2♦    =    3+ ♦

other    =    natural, says nothing about ♦