18. Trade and other receivables (in millions of euros)December 31, 2016December 31, 2015Receivables from clients from projects in progress11Trade receivables228227Unbilled revenue202106Other taxes and social security charges2333Other receivables270292Total724659'Trade and other receivables' includes an amount of €104 million (€16 million in 2015) that concerns related parties.The combined credit risk exposure from trade receivables and other receivables (excluding the construction contract work in progress) and the impairment losses are shown in note 30.The ‘Other receivables’ show the current part of the receivable (€120 million) payable by the Railway Pensions Fund and the current part of the receivable connected to correction mechanism concession fee (€15 million).Work in progress(in millions of euros)December 31, 2016December 31, 2015Costs of work in progress1022Realised gains and losses-1-2 920Less: billed instalments1223 -3-3Presented under: Receivables from clients for projects in progress11Advance payments received for projects in progress-4-4The trade payables and other liabilities are stated in note 27.