Hugh Hefner recently with Paris Hilton

Paris Hilton has been invited to pose for Playboy, but her mother won't agree. Playboy millionaire Hugh Hefner, who would love the hotel heiress to pose for the magazine when she is released from jail, feels Kathy's disapproval will prevent her. Hefner told Access Hollywood: "Well, that's a standing offer. I think the problem quite frankly is mama. Mama doesn't want her to pose."Meanwhile, the 26-year-old has been spending her time behind bars reading fan mail.'The Simple Life' star has even been replying to the letters that touch her most. In a handwritten note obtained by E! Online, Paris wrote: "I read your letter and just wanted to thank you for your kind words of love and support.The fact that you took time out of your day to write me truly means the world. Especially at such a difficult and scary time in my life."But I am being strong and trying to make the best out of the situation."And the letters I'm receiving really do put a smile on my face as I sit here in my cell, sad and alone. Again, thank you so much and may God bless you and your family."Love always, Paris Hilton."She signed the letter by dotting her i's with hearts and adding "xoxo". Jail is said to be doing wonders for Paris' complexion. Actress Julie Araskog - who visited the socialite at the Lynwood jail on Saturday along with her sister Nicky Hilton - was amazed by how good Paris looked. Araskog told People magazine: "She looked so beautiful."It felt surreal, like a movie. No make-up - she is naturally beautiful. I was stunned."She was as beautiful as she is with make-up."