Christina Hendricks (left) and January Jones
's January Jones thinks costar Christina Hendricks looks great on her new diet – but hopes it doesn't cause her to lose one of her best assets.
"I think she looks beautiful. Her body is amazing," Jones, 31, said at the Entertainment Tonight
/PEOPLE Emmy after-party. "My only advice is to not go too far, because if she loses her curves it will be a very sad day."
But as for what soon-to-be-wed
Hendricks's regimen is, Jones can't help you – it's still a secret to her.
"I know that Christina has gone on a diet," she said. "She already looks amazing, but you know, before you get married you want to look the absolute best. I don't know what she's doing."
She's also not sure what she'll be getting as a wedding gift for either Hendricks or her other costar Elisabeth Moss, who is engaged
to Saturday Night Live
funnyman Fred Armisen.
"I don't know … a check? No, I don't know," she said. "They're both really different. I'll have to go on their registry."