Just how open is Android’s “open” system?

Google Inc. GOOG -0.29% says its Android mobile operating system is “open.” But newly discovered documents reveal the strings attached. The documents show that Google has imposed strict restrictions on device makers that want access to its search engine, YouTube or the more than one million apps in its Play Store. In return, the device makers must feature other Google apps and set Google search as the default for users, according to the agreements. European antitrust authorities are examining those conditions, among others, as they consider whether Google has abused Android’s position as the leading smartphone operating system. In Europe, companies with dominant market share are required to promote competition, said Ioannis Lianos, a professor at University College London who studies competition law.