One of France’s most prolific serial killers goes on trial this week, but the spotlight will be on his wife after revelations that she made a “diabolical pact” to help him murder young women.

The court will hear evidence of how Monique Olivier, 59, helped Michel Fourniret, the so-called “Ogre of the Ardennes”, to lure young women into a van so that he could kidnap and rape them.

A well spoken, bespectacled figure with a love of Russian literature, Fourniret, 65, is being tried for the murders of seven young women. His wife is charged with one murder and as an accomplice in six others: she has told police that she witnessed various rapes and murders and has admitted to gagging one girl to silence her screams.

France has been sickened by the case, particularly Olivier’s suggestions that the couple’s 20-year-old son Sélim witnessed rapes and murders when he was young. He will testify during the trial, as will Fourniret’s first two wives.

Investigators believe that in addition to the seven murders the couple have acknowledged, Fourniret may have killed several other women, including Joanna Parrish, a British student teacher whose body was found in a river in 1990, and three of his son’s babysitters.

Fourniret denies killing Parrish, but Olivier has described the murder in Burgundy of a young woman bearing a resemblance to Parrish. She met her killer after placing an advertisement for English language teaching and babysitting in a magazine. Fourniret often consulted advertisements for babysitters. He will be investigated for her murder after the trial.

In her chilling confession to police, Olivier, who speaks with a stammer, has described a “habitual scenario” of driving around in the countryside, “hunting for virgins”. She acted as a decoy to gain the confidence of the victims.

Fourniret would sometimes ask her to examine girls before raping them to make sure that they were virgins.

The couple met when Fourniret advertised for a pen pal after being imprisoned in 1984 for sexual assaults in the Paris region.

They began a long correspondence and she fell under his sway, lovingly addressing him as “my beast” or as “Shere Khan”, after the tiger in Kipling’s Jungle Book. He called her “Natouchka”.

According to The Pact of the Fournirets, a book by two journalists that appeared last week, Fourniret offered to help Olivier, a former care worker, to get even with two exhusbands, whom she claimed had abused her, as soon as he got out of prison. In exchange she agreed to help him find virgins, or what Fourniret referred to as “membranes on legs”.

When he was freed in 1987, Olivier was waiting in her green Peugeot estate. Fourniret never kept his part of the bargain by killing her former husbands. Less than two months after his release, however, Olivier made what she described as her first offering to her “beast”.

Isabelle Laville was a timid 17-year-old who was undergoing psychotherapy. Olivier said she had selected her because she looked like a younger version of herself: her husband wanted to imagine that he was deflowering Olivier.

When the car drew up alongside Laville, Olivier asked for directions. She persuaded the girl to get in and direct her.

Waiting down the road with a jerry can was Fourniret, posing as a motorist who had broken down and needed a lift. Olivier stopped the car and let him in.

As they drove off, Fourniret, sitting in the back, put a rope around Laville’s neck and told her she was his prisoner. The girl was given an overdose of sleeping pills on the way back to the couple’s home.

When they got there, Olivier had sex with Fourniret when he found that he was unable to rape the victim. Fourniret then strangled Laville. In 2006 he led police to her remains in a disused well.

Olivier said that she had not known Fourniret was going to kill Laville. Whatever the case, the murder seems only to have cemented their relationship.

In 1988, a heavily pregnant Olivier approached Fabienne Leroy, a 20-year-old student, telling her that she urgently needed to get to a doctor.

Leroy got into the car. This time Fourniret was driving.

He drove to a field and pulled Leroy out of the car at gunpoint. He raped and shot her after Olivier had subjected her to an intimate examination. Leroy’s body was found in the field the next day. Several more murders followed the same pattern.

Fourniret’s mother had once worked as a servant in a chateau and psychologists believe that this was one of the reasons he bought a chateau in the Ardennes with funds stolen from one of his former prison cellmates.

The couple lived in one sparsely furnished corner of the vast property. Fourniret buried some of the bodies in the grounds.

In one case the couple were believed to have used little Sélim as bait, telling their victim that they were looking for a doctor for their child. On other occasions they left the child with a babysitter when going off on “virgin-hunting” weekends.

Even if Olivier did not always accompany Fourniret on his expeditions, he made a point of telling her about his adventures when he got home. “I went hunting,” was how he would put it, or, “I obtained satisfaction.”

In 2003, however, Fourniret slipped up. A 13-year-old girl whom he had kidnapped in Belgium managed to escape from his van when he stopped at a petrol station.

She was rescued by a passing motorist who took down Fourniret’s registration number. He was arrested. Several human hairs were found in the van, one of them belonging to Mananya Thumpong, 13, whose remains had been found in a forest in 2001.

When Olivier decided to talk, Fourniret soon went back on his denials. He led police to various bodies, conducting excavations in the grounds of his chateau in a bulletproof jacket.

The trial is expected to last up to two months, after which Fourniret, if found guilty, will spend the rest of his life behind bars. And France expects no less a prison sentence for his “ogress”.

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