Chelsea have signed Senegal defender Papy Djilobodji from Nantes in a £4m deal and Michael Hector, also for £4m, from Championship side Reading.
Djilobodji, 26, has agreed a four-year contract at Stamford Bridge.
Chelsea manager Jose Mourinho turned his attention to the 6ft 4in centre-back after failing to sign England's John Stones from Everton.
Centre-back Hector, 23, has agreed a five-year deal but will spend the rest of the season at the Championship club.
|More on Transfer Deadline Day|
|Man Utd deny blame for failed Real Madrid deal||Southampton sign Celtic defender Van Dijk for £11.5m|
|Where has the last 48 hours left Man Utd?||Transfer deadline day - done deals|
Djilobodji was in the last year of his contract and had made 183 appearances for Nantes after joining them in 2010.
"I'm very happy to sign for a big club like Chelsea and I want to do my best to help the team win titles. I want to thank the club for giving me this opportunity," he said.
Mourinho has moved to address the Premier League champions' defensive concerns after they conceded nine goals in their first four league games.
|Fan's view: Ross Mooring, Chelsea supporter|
|"If you had told Chelsea fans that before the end of transfer deadline day their club had bought two new players they would struggle to contain broad smiles on their faces. However, the names Papy Djilobodji and Michael Hector have caused more than a few quizzical looks instead.|
|"Both in their mid-twenties and both signed for less than some of the Chelsea youth products at centre-back are probably worth; namely full internationals Andreas Christensen, Tomas Kalas and Kenneth Omeruo.|
|"More and more the realisation is setting in that Chelsea's squad policy consists of high-profile signings, buy-to-loans and academy afterthoughts. The loan count? 33!"|
That has left them eight points behind leaders Manchester City before they return to action at Everton on 12 September.
Jamaica international Hector, a boyhood Chelsea supporter, began his career in the youth team at Millwall and moved to Reading's academy in 2009.
Earlier in the day, Chelsea agreed a new four-year deal with winger Victor Moses, 24, and sent him on loan to West Ham for the season.
Find all the latest football transfers on our dedicated page.