It's a subject not often written, and almost never well, this story of the older woman and the younger man. Only a few times has the tale itself been successful or even plausible -- I think of a novel written a few years back, called A MUCH YOUNGER MAN.

And even that was slightly unsavory, as the "attraction" began when the thirtysomething woman in question became the crush of a 15-year-old boy, the son of her best friend. It smacked a bit of the middle-school teacher and her hip-hop sweetheart to me.

In A MUCH YOUNGER MAN, out of decorum (and with an eye toward rape charges) they waited until the lad turned 18 before slipping between the sheets. When they did, it was much to the ire of the boy's mother, and you know, as the mother of teenage sons, I can't blame her, delicately as the story was told. They then slipped off to another country, the same country in which a similar but very different story, MADEMOISELLE BENOIR, is told.

MADEMOISELLE BENOIR (Houghton Mifflin) by Christine Conrad proves that a story such as this need not be the equivalent of the joke about a dog that stands on its hind legs -- it is not that the thing is done well, it's that it's done at all.

Mlle. Benoir takes far too many chances and gets away with all of them.

First, it's an epistolary novel, a dicey proposition at best, often as ho-hum as reading more than a page of someone else's diary. But the letters in MADEMOISELLE BENOIR crackle as they describe a love story of depth and power, between a young American artist aged 30 and an aristocratic Frenchwoman 20 years older.

That is, she is 50, the dread age no woman is ever supposed to attain and still retain a woman's desires and a woman's allure -- even today, when we're "past all that."

Fifty may be the new forty, but it's not the new thirty. As a man of my acquaintance put it -- very delicately -- when a 36-year-old pal of his fell in love with a 59-year-old woman, "I mean, she's gorgeous, but it's just.. disgusting."

This same acquaintance, I'm at pains to add, cheers his own father for having, at 60, a 35-year-old wife.

Nature still abhors a role reversal -- male nature, that is. And mabye female, too. Women can be as mysognist as the next NASCAR fan. Another acquaintance of mine told me recently about a fellow who dumped his intended (twelve years older) literally at the altar. "I mean, really, though," she said. "Did she ever dream he'd go through with it? What could a man his age have possibly seen in a woman her age?"

Welll, in Conrad's lovely, lively novel, apparently quite a good deal.

The young American, Tim, even gives up a comely bride of tender years not just to woo but to marry the "spinster" Mademoiselle Benoir, the elder of two sisters of an ancient French family. He is very much the whiz-bang American boy, and she is very much the French swan; and yet she accepts his admiration as her due, and returns his passion -- although, in an absolutely charming twist, Mlle Benoir's Catholicism requires that they wait until their wedding night to consummate their love.

Witty, comely Catherine makes her parish pere blush when he praises what he assumes will be a platonic marriage for its recogniton of love on a higher plane. She confesses she woudln't be interested in love of that sort at all.

And though Tim's extraordinary parents are taken aback and full of cautionary remarks, and Catherine's sister flies into an absolute rage, and though even Catherine has moments of tristesse when she recognizes that she quite likely will not be able to give Tim children (not a wish that's on top of the stack of hankies in his drawer), the couple perseveres, despite knowing that her body will change more quickly than his, that he might outlive her, that there will be sniggers and suggestions about brash gold-diggers on gilded soil.

As Conrad describes it, the relationship is one of mutual respect and ineffable affection, which there is no reason to suspect that time will tarnish.

It's an interesting and touching novel, that works despite its quiet pace and unusual form, just as does the marriage of Mademoiselle Benoir.

And just as the marriage of Madame Mitchard does, eight years after she married a man twelve years younger than she.