It’s hard to imagine the elegant 61-year-old sitting before me taking part in a Parisian orgy. Chic and properly dressed, she looks like the French art critic she is — only a series of black-and-white nude photographs of her on the sitting-room wall hints at sexual adventures with an incalculable number of partners.

She had sex in the Bois de Boulogne, swingers’ clubs, offices and cemeteries. If there was a sex party taking place in Paris, you can bet Catherine Millet was there, and when she unemotionally wrote it all down in a memoir, The Sexual Life of Catherine M, it was an international bestseller. What, then, can the doyenne of free love really have to say about the thorny subject of jealousy in a committed relationship? Not much, you might think, yet her new book, Jealousy: The Other Life of Catherine M, is all about the breakdown she suffered when she discovered her husband had also been having affairs. She’s aware of the irony. “The moral of the story is that you can have all sorts of beliefs about how you should live your life, but these can be completely knocked down by drive and passion.”

In her case, the crisis came after 15 years with Jacques Henric, a writer and photographer. Up until then, they had both been happily seeing other people; the key to the success of their arrangement was discretion. “Jacques always had other girlfriends, just as I had other men. When I was promoting my book, many couples asked us how we could be so happy together yet live such a sexually free life. Our secret was, we just didn’t talk about it. We talked about everything else — politics, art, literature — but never our own relationship. We also led quite separate lives, and when you don’t want to see things, you are blind.”

That changed when Millet found a photograph of one of Henric’s lovers, who just happened to be young and beautiful. In the age-old fashion of the betrayed wife, she turned detective, hunting through his notebooks, watching his every move. She was gripped by obsessional fantasies about Henric and other women, and soon was unable to experience sexual pleasure without them. She had panic attacks, took tranquillisers. The crisis lasted three years. “This jealousy was started by the photographs,” she says. “It was the first time I had been able to see another woman in his life, and she was naked. This had a much more powerful effect on me than any letters I had found before.”

Of course, Millet couldn’t be angry, given her lifestyle. Henric comforted her, but he wasn’t about to change; nor did she ask him to. She began to feel trapped and desperate and, in the end, as the French do, she went back to her psychoanalyst. “It was then that I realised I was taking enormous masochistic pleasure in the jealousy, and particularly in the fantasies I conjured up of Jacques and other women. I had become a voyeur.”

Millet never contemplated leaving Henric, or that he might leave her. “Even though I felt intensely jealous of Jacques and the women, I never thought it was about love: I saw it as purely sexual.” Since the book’s publication in France, Millet has been bombarded with stories of jealousy from readers. “For women, jealousy is about the desire to possess another person; for men, it’s more about competition. Men are going to think, ‘Is this man giving my wife more pleasure than I can?’ Jealousy is unavoidable, but it can be positive: the sex between Jacques and me was much more intense when I was jealous.” Millet believes it is rare to come across a long-term couple who have both a strong, loving relationship and a fantastic sex life. “There is always a moment when one of the partners is tempted. When that happens, the big lessons are discretion and to be patient. Patience, I would say, is one of the great signs of love.”

For the woman who became known as “Madame Sex”, wild orgies are a thing of the past, as is any sex outside marriage — she is, she says, monogamous now. Not, she points out, that she thinks that is necessarily a better way, it just feels right at the moment. And Henric? Is he faithful? A Gallic shrug.“I don’t know.” And you can tell she doesn’t ask.

How I found out

As was his habit, Jacques had gone out to buy the newspapers. I needed an envelope. I found one in a bundle on a shelf near his desk. When I opened it, I noticed it contained a piece of paper. Obviously I should never have taken out the piece of paper, as Jacques repeatedly pointed out afterwards, but it had become an automatic reflex. The first line said: “I’m tearing out this page from my diary because I know Catherine reads it. This must not fall into her hands.”

I read that he is at Le Pradié, the house where our friend Bernard lives, in the Aveyron. Blandine has gone with him, and another friend of hers. He jokes about the way the two of them, mature men, are titillated by the presence of the two young women. They take photos of them, the girls undress at their request. Blandine shares his bed, but refuses to have sex. Even so, he is able to caress her, and she falls asleep wrapped around him. I begin to tremble so much that I had great difficulty holding the pen with which I immediately wrote to Jacques. I must have scribbled more or less the same words as the ones with which I attempted to ward off the crises: ‘Jacques... please... It hurts too much... Help me...” As my legs had gone from under me, I went and lay down on the bed. I had not trembled like this, all over, since the day I received a call from the police station, asking me to “come as quickly as possible” because my mother had had “an accident”. At my insistence, the voice eventually informed me that she had died.

© Catherine Millet 2009

Jealousy is published by Profile on Thursday at £10.99. Buy it for £9.89, including p&p, from BooksFirst on 0845 271 2135 or at timesonline.co.uk/booksfirst

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