Nokia's had a rough time with its high-end Windows Phones over the years, but it's excelled with its cheaper handsets that cater to emerging markets and anyone interested in a budget smartphone. Announced today at Build, the Lumia 630 and 635 focus on expanding the latter category. The new devices come with Windows Phone 8.1 onboard, and while the 635 includes speedy LTE data, the 3G-only 630 will also come in a dual-SIM variant. They'll be available in five colors with changeable shells, and even offer features like fitness tracking with an integrated low-power chip called SensorCore.

They'll arrive in May as the first Lumia devices with Windows 8.1, running on quad-core 1.2GHz Snapdragon 400 processors. As one would expect, these will ship in Asia first before spreading out worldwide, with a US release planned for July and T-Mobile / MetroPCS already announced as a carrier. Off-contract, the Lumia 630 will cost $159 / $169 for the single- or dual-SIM versions, while the 635 will be $189. We should know more about these and get hands-on experience in a few hours when Nokia holds its own press conference, so check back then for more details.