Stars NL50 Flush Draw vs Underbetting

[AlexM]

Villain is unknown. Due to his underbets I thought calling was the right move, but I'm not sure and I have no idea what I'd have done if I hit an A or a K. Comments?

Poker Stars
No Limit Holdem Ring game
Blinds: $0.25/$0.50
9 players
Converter

Stack sizes:
UTG: $22.90
UTG+1: $8
MP1: $159.65
MP2: $79
MP3: $51.75
CO: $48.25
Hero: $49.25
SB: $31.95
BB: $29.50

Pre-flop: (9 players) Hero is Button with K[img]/images/graemlins/diamond.gif[/img] A[img]/images/graemlins/diamond.gif[/img]
4 folds, <font color="#cc0000">MP3 raises to $2</font>, CO folds, <font color="#cc0000">Hero raises to $6.5</font>, 2 folds, MP3 calls.

Flop: 9[img]/images/graemlins/heart.gif[/img] 2[img]/images/graemlins/diamond.gif[/img] 6[img]/images/graemlins/diamond.gif[/img] ($13.75, 2 players)
<font color="#cc0000">MP3 bets $7</font>, Hero calls.

Turn: 5[img]/images/graemlins/heart.gif[/img] ($27.75, 2 players)
<font color="#cc0000">MP3 bets $12</font>, Hero calls.

River: 7[img]/images/graemlins/heart.gif[/img] ($51.75, 2 players)
<font color="#cc0000">MP3 is all-in $26.25</font>, Hero folds.
Uncalled bets: $26.25 returned to MP3.

Results:
Final pot: $51.75