The ruling by the Paris Appeal Court states that Jérôme Kerviel has been placed in temporary detention. In the French justice system, temporary can mean a long time.

Overworked investigating magistrates often take months to complete inquiries in relatively simple cases, and the rogue trader scandal at Société Générale is anything but simple. Renaud van Ruymbeke and Françoise Desset, the magistrates heading the investigation, may need a year or two to dissect the process that led Mr Kerviel to bet €50 billion (£37.3 billion) and lose €5 billion at SocGen, judicial sources say.

The worst-case scenario for Mr Kerviel would be to stay in La Santé prison in Paris for the duration of their investigation. It would be a grim prospect for a young man who, until last month, dreamt of earning billions of euros for his bank and hundreds of thousands for himself. La Prison de la Santé means the Prison of Health, but the name is highly inappropriate. The sombre stone building dates from 1867 and is known for its overcrowding, its filth and the echoing howls of deranged inmates.

If Mr Kerviel is fortunate, he will be placed in the so-called VIP wing, where Bernard Tapie, the businessman and politician, and Jean-Christophe Mitterand, the son of François Mitterrand, the former President, have spent time. There, he would get a cell to himself. But even so, the conditions for VIPs are only slightly less appalling than those in the rest of the jail, according to former prisoners. Many say that you are lucky to escape with your mental health after only months in La Santé.

Elisabeth Meyer, Mr Kerviel's lawyer, said that she would appeal for him to be released on bail on Monday. This is a long shot. She has a better chance of winning bail for him in a few months if she can demonstrate that the investigation has moved beyond the point where his further detention might be considered necessary. In the meantime, Mr Kerviel will have to prepare his defence from behind bars. The risk for him is that he could come to be seen as a culprit, which is what SocGen claims him to be, rather than a victim, which is how he has been portrayed on the internet and in some of the media.

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