In case anyone is interested, among the documentation preserved in the Saxe-Coburg-Gotha family archive, which is preserved in the Austrian State Archives in Vienna, there's the correspondence of Clementine d'Orleans, daughter of King Louis Philippe and mother of King Ferdinand I of Bulgaria. She kept correspondence with several royals around Europe, including members of the Brazilian, French, Belgian, Portuguese, British and Bulgarian royal families.
Among the royals who regularly wrote to Clementine, there were Queen Maria II of Portugal, Queen Marie Amelie of Portugal, Queen Marie Henriette of Belgium, Crown Princess Stephanie of Austria, Princess Louise of Belgium, Alexandrine of Baden (Duchess of Saxe-Coburg-Gotha), Ferdinand I of Bulgaria, his first wife Marie Louise of Bourbon-Parma, Princess Helena of the United Kingdom, etc.
There is a huge amount of letters from Queen Victoria to Clementine preserved in this archive. It seems that they were good friends since they wrote to each other during half a century.