The snails and curled-up cats that decorate the margins of Édouard Manet’s letters are admired for their effortless spontaneity. But an art historian has discovered that the artist recycled these enchanting watercolour doodles by tracing them from designs he had prepared earlier.

By comparing Manet’s illustrated letters with his sketchbook drawings, all thought to date from 1880, Dr Emily Beeny has realised a “reuse of motifs” with which he created “an illusion of effortlessness”. She believes that he made “extensive use of tracing” in an apparent attempt to “conceal the effort … required to produce these ‘spontaneous’ works”.

Beeny, associate curator of drawings at the J Paul Getty Museum, told the Observer that she has found numerous cases of tracing, with women’s ankles emerging from ruffled hemlines and a watering-can nestled in the grass among the images: “He uses the same sketchbook drawing as the basis for watercolours in letters to multiple recipients. With the crudest of PowerPoint tools, you can overlay a Jpeg of one over another and see that he follows the contours. This idea of tracing has never been published.”

Manet’s sketchbook study of legs: the dominant lines are very similar to those in the above letter to Madame Guillemet.

She said of the illustrated letters: “They are always described as breezy, virtuosic and effortless, and dashed off in a moment. But what I discovered is that most of these things seem to have been traced from more searching and careful drawings that he’d made in his sketchbooks. He would take semi-transparent letter paper, lay it down over a sketchbook page, trace that design with a wash of grey watercolour and then basically colour it in with watercolour.”

Manet once said that artists should capture what they see “at the first go”. Often cited as the father of the impressionists, his masterpieces include Le Déjeuner sur l’herbe,Olympia and A Bar at the Folies-Bergère.

In letters to two different friends, he included depictions of his cat, Zizi, telling one of them: “Here is Zizi coming to pose for me as if she wanted me to give you her news, since I’m writing from my garden.” Beeny observes that “the charming fiction of the cat posing in the garden makes plain Manet’s hope that such drawings would be received as fresh and spontaneous … unpremeditated, easy”. Overlays of the initial sketch with the two letters tell a different story, she adds.

A letter to Madame Jules Guillemet, one of Manet’s favourite models, is “cheekily strewn with pretty ankles and ruffled hemlines”. Beeny says: “This letter is written on transparent wove paper . The succinct and playful nonchalance of Manet’s designs suggests that he sketched these ankles on the fly – and perhaps even on the sly – under cafe tables and on sidewalks. In the case of one pair of ankles, however, a revised sketch of the same motif survives, revealing a somewhat more careful method.”

Asked how the tracings could have been overlooked until now, she suggested a possible reluctance among specialists to “diminish our image of Manet as someone with effortless mastery over his brush”.

She made the discovery while preparing a major Getty exhibition on the artist, which she has co-curated. She will publish her findings in the December issue of the scholarly Burlington Magazine. In that essay, she concludes: “While the breezy impression these works create is universally admired, the furtive effort by means of which Manet achieved this impression is less easy to accept, forcing us, as it does, to confront his technical limitations … and perhaps also his reputational anxieties.”

The exhibition, titled Manet and Modern Beauty and organised in association with the Art Institute of Chicago, reunites several of these watercolours with the sketchbook drawings from which they were traced, is on until 12 January.